* command/jsonstate: remove redundant remarking of resource instance
ResourceInstanceObjectSrc.Decode already handles marking values with any marks stored in ri.Current.AttrSensitivePaths, so re-applying those marks is not necessary.
We've gotten reports of panics coming from this line of code, though I have yet to reproduce the panic in a test.
* Implement test to reproduce panic on #29042
Co-authored-by: David Alger <davidmalger@gmail.com>
Because our snippet generator is trying to select whole lines to include
in the snippet, it has some edge cases for odd situations where the
relevant source range starts or ends directly at a newline, which were
previously causing this logic to return out-of-bounds offsets into the
code snippet string.
Although arguably it'd be better for the original diagnostics to report
more reasonable source ranges, it's better for us to report a
slightly-inaccurate snippet than to crash altogether, and so we'll extend
our existing range checks to check both bounds of the string and thus
avoid downstreams having to deal with out-of-bounds indices.
For completeness here I also added some similar logic to the
human-oriented diagnostic formatter, which consumes the result of the
JSON diagnostic builder. That's not really needed with the additional
checks in the JSON diagnostic builder, but it's nice to reinforce that
this code can't panic (in this way, at least) even if its input isn't
valid.
* terraform: use hcl.MergeBodies instead of configs.MergeBodies for provider configuration
Previously, Terraform would return an error if the user supplied provider configuration via interactive input iff the configuration provided on the command line was missing any required attributes - even if those attributes were already set in config.
That error came from configs.MergeBody, which was designed for overriding valid configuration. It expects that the first ("base") body has all required attributes. However in the case of interactive input for provider configuration, it is perfectly valid if either or both bodies are missing required attributes, as long as the final body has all required attributes. hcl.MergeBodies works very similarly to configs.MergeBodies, with a key difference being that it only checks that all required attributes are present after the two bodies are merged.
I've updated the existing test to use interactive input vars and a schema with all required attributes. This test failed before switching from configs.MergeBodies to hcl.MergeBodies.
* add a command package test that shows that we can still have providers with dynamic configuration + required + interactive input merging
This test failed when buildProviderConfig still used configs.MergeBodies instead of hcl.MergeBodies
This PR adds decoding for the upcoming "moved" blocks in configuration. This code is gated behind an experiment called EverythingIsAPlan, but the experiment is not registered as an active experiment, so it will never run (there is a test in place which will fail if the experiment is ever registered).
This also adds a new function to the Targetable interface, AddrType, to simplifying comparing two addrs.Targetable.
There is some validation missing still: this does not (yet) descend into resources to see if the actual resource types are the same (I've put this off in part because we will eventually need the provider schema to verify aliased resources, so I suspect this validation will have to happen later on).
Previously, if any resources were found to have drifted, the JSON plan
output would include a drift entry for every resource in state. This
commit aligns the JSON plan output with the CLI UI, and only includes
those resources where the old value does not equal the new value---i.e.
drift has been detected.
Also fixes a bug where the "address" field was missing from the drift
output, and adds some test coverage.
* command: new command, terraform add, generates resource templates
terraform add ADDRESS generates a resource configuration template with all required (and optionally optional) attributes set to null. This can optionally also pre-populate nonsesitive attributes with values from an existing resource of the same type in state (sensitive vals will be populated with null and a comment indicating sensitivity)
* website: terraform add documentation
* Quoting filesystem path in scp command argument
* Adding proper shell quoting for scp commands
* Running go fmt
* Using a library for quoting shell commands
* Don't export quoteShell function
* jsonplan and jsonstate: include sensitive_values in state representations
A sensitive_values field has been added to the resource in state and planned values which is a map of all sensitive attributes with the values set to true.
It wasn't entirely clear to me if the values in state would suffice, or if we also need to consult the schema - I believe that this is sufficient for state files written since v0.15, and if that's incorrect or insufficient, I'll add in the provider schema check as well.
I also updated the documentation, and, since we've considered this before, bumped the FormatVersions for both jsonstate and jsonplan.
An unknown block represents a dynamic configuration block with an
unknown for_each value. We were not catching the case where a provider
modified this value unexpectedly, which would crash with block of type
NestingList blocks where the config value has no length for comparison.
Historically, we've used TFC's default run messages as a sort of dumping
ground for metadata about the run. We've recently decided to mostly stop
doing that, in favor of:
- Only specifying the run's source in the default message.
- Letting TFC itself handle the default messages.
Today, the remote backend explicitly sets a run message, overriding
any default that TFC might set. This commit removes that explicit message
so we can allow TFC to sort it out.
This shouldn't have any bad effect on TFE out in the wild, because it's
known how to set a default message for remote backend runs since late 2018.
* tools: remove terraform-bundle.
terraform-bundle is no longer supported in the main branch of terraform. Users can build terraform-bundle from terraform tagged v0.15 and older.
* add a README pointing users to the v0.15 branch
Previously we had a separation between ModuleSourceRemote and
ModulePackage as a way to represent within the type system that there's an
important difference between a module source address and a package address,
because module packages often contain multiple modules and so a
ModuleSourceRemote combines a ModulePackage with a subdirectory to
represent one specific module.
This commit applies that same strategy to ModuleSourceRegistry, creating
a new type ModuleRegistryPackage to represent the different sort of
package that we use for registry modules. Again, the main goal here is
to try to reflect the conceptual modelling more directly in the type
system so that we can more easily verify that uses of these different
address types are correct.
To make use of that, I've also lightly reworked initwd's module installer
to use addrs.ModuleRegistryPackage directly, instead of a string
representation thereof. This was in response to some earlier commits where
I found myself accidentally mixing up package addresses and source
addresses in the installRegistryModule method; with this new organization
those bugs would've been caught at compile time, rather than only at
unit and integration testing time.
While in the area anyway, I also took this opportunity to fix some
historical confusing names of fields in initwd.ModuleInstaller, to be
clearer that they are only for registry packages and not for all module
source address types.
We have some tests in this package that install real modules from the real
registry at registry.terraform.io. Those tests were written at an earlier
time when the registry's behavior was to return the URL of a .tar.gz
archive generated automatically by GitHub, which included an extra level
of subdirectory that would then be reflected in the paths to the local
copies of these modules.
GitHub started rate limiting those tar archives in a way that Terraform's
module installer couldn't authenticate to, and so the registry switched
to returning direct git repository URLs instead, which don't have that
extra subdirectory and so the local paths on disk now end up being a
little different, because the actual module directories are at a different
subdirectory of the package.
Now that we (in the previous commit) refactored how we deal with module
sources to do the parsing at config loading time rather than at module
installation time, we can expose a method to centralize the determination
for whether a particular module call (and its resulting Config object)
enters a new external package.
We don't use this for anything yet, but in later commits we will use this
for some cross-module features that are available only for modules
belonging to the same package, because we assume that modules grouped
together in a package can change together and thus it's okay to permit a
little more coupling of internal details in that case, which would not
be appropriate between modules that are versioned separately.
It's been a long while since we gave close attention to the codepaths for
module source address parsing and external module package installation.
Due to their age, these codepaths often diverged from our modern practices
such as representing address types in the addrs package, and encapsulating
package installation details only in a particular location.
In particular, this refactor makes source address parsing a separate step
from module installation, which therefore makes the result of that parsing
available to other Terraform subsystems which work with the configuration
representation objects.
This also presented the opportunity to better encapsulate our use of
go-getter into a new package "getmodules" (echoing "getproviders"), which
is intended to be the only part of Terraform that directly interacts with
go-getter.
This is largely just a refactor of the existing functionality into a new
code organization, but there is one notable change in behavior here: the
source address parsing now happens during configuration loading rather
than module installation, which may cause errors about invalid addresses
to be returned in different situations than before. That counts as
backward compatible because we only promise to remain compatible with
configurations that are _valid_, which means that they can be initialized,
planned, and applied without any errors. This doesn't introduce any new
error cases, and instead just makes a pre-existing error case be detected
earlier.
Our module registry client is still using its own special module address
type from registry/regsrc for now, with a small shim from the new
addrs.ModuleSourceRegistry type. Hopefully in a later commit we'll also
rework the registry client to work with the new address type, but this
commit is already big enough as it is.
We've previously had the syntax and representation of module source
addresses pretty sprawled around the codebase and intermingled with other
systems such as the module installer.
I've created a factored-out implementation here with the intention of
enabling some later refactoring to centralize the address parsing as part
of configuration decoding, and thus in turn allow the parsing result to
be seen by other parts of Terraform that interact with configuration
objects, so that they can more robustly handle differences between local
and remote modules, and can present module addresses consistently in the
UI.
This new package aims to encapsulate all of our interactions with
go-getter to fetch remote module packages, to ensure that the rest of
Terraform will only use the small subset of go-getter functionality that
our modern module installer uses.
In older versions of Terraform, go-getter was the entire implementation
of module installation, but along the way we found that several aspects of
its design are poor fit for Terraform's needs, and so now we're using it
as just an implementation detail of Terraform's handling of remote module
packages only, hiding it behind this wrapper API which exposes only
the services that our module installer needs.
This new package isn't actually used yet, but in a later commit we will
change all of the other callers to go-getter to only work indirectly
through this package, so that this will be the only package that actually
imports the go-getter packages.
As the comment notes, this hostname is the default for provide source
addresses. We'll shortly be adding some address types to represent module
source addresses too, and so we'll also have DefaultModuleRegistryHost
for that situation.
(They'll actually both contain the the same hostname, but that's a
coincidence rather than a requirement.)
When performing state migration to a remote backend target, Terraform
may fail due to mismatched remote and local Terraform versions. Here we
add the `-ignore-remote-version` flag to allow users to ignore this
version check when necessary.
When migrating multiple local workspaces to a remote backend target
using the `prefix` argument, we need to perform the version check
against all existing workspaces returned by the `Workspaces` method.
Failing to do so will result in a version check error.
Our module installer has a somewhat-informal idea of a "module package",
which is some external thing we can go fetch in order to add one or more
modules to the current configuration. Our documentation doesn't talk much
about it because most users seem to have found the distinction between
external and local modules pretty intuitive without us throwing a lot of
funny terminology at them, but there are some situations where the
distinction between a module and a module package are material to the
end-user.
One such situation is when using an absolute rather than relative
filesystem path: we treat that as an external package in order to make the
resulting working directory theoretically "portable" (although users can
do various other things to defeat that), and so Terraform will copy the
directory into .terraform/modules in the same way as it would download and
extract a remote archive package or clone a git repository.
A consequence of this, though, is that any relative paths called from
inside a module loaded from an absolute path will fail if they try to
traverse upward into the parent directory, because at runtime we're
actually running from a copy of the directory that's been taking out of
its original context.
A similar sort of situation can occur in a truly remote module package if
the author accidentally writes a "../" source path that traverses up out
of the package root, and so this commit introduces a special error message
for both situations that tries to be a bit clearer about there being a
package boundary and use that to explain why installation failed.
We would ideally have made escaping local references like that illegal in
the first place, but sadly we did not and so when we rebuilt the module
installer for Terraform v0.12 we ended up keeping the previous behavior of
just trying it and letting it succeed if there happened to somehow be a
matching directory at the given path, in order to remain compatible with
situations that had worked by coincidence rather than intention. For that
same reason, I've implemented this as a replacement error message we will
return only if local module installation was going to fail anyway, and
thus it only modifies the error message for some existing error situations
rather than introducing new error situations.
This also includes some light updates to the documentation to say a little
more about how Terraform treats absolute paths, though aiming not to get
too much into the weeds about module packages since it's something that
most users can get away with never knowing.
When returning from the context method, a deferred function call checked
for error diagnostics in the `diags` variable, and unlocked the state if
any exist. This means that we need to be extra careful to mutate that
variable when returning errors, rather than returning a different set of
diags in the same position.
Previously this would result in an invalid plan file causing a lock to
become stuck.
Most legacy provider resources do not implement any import functionality
other than returning an empty object with the given ID, relying on core
to later read that resource and obtain the complete state. Because of
this, we need to check the response from ReadResource for a null value,
and use that as an indication the import id was invalid.
This was dead code, and there is no clear way to retrieve this
information, as we currently only derive the drift information as part
of the rendering process.
The schemas for provider and the resources didn't match, so the changes
were not going to be rendered at all.
Add a test which contains a deposed resource.
* getproviders ParsePlatform: add check for invalid platform strings with too many parts
The existing logic would not catch things like a platform string containing multiple underscores. I've added an explicit check for exactly 2 parts and some basic tests to prove it.
* command/providers-lock: add tests
This commit adds some simple tests for the providers lock command. While adding this test I noticed that there was a mis-copied error message, so I replaced that with a more specific message. I also added .terraform.lock.hcl to our gitignore for hopefully obvious reasons.
getproviders.ParsePlatform: use parts in place of slice range, since it's available
* command: Providers mirror tests
The providers mirror command is already well tested in e2e tests, so this includes only the most absolutely basic test case.
Do not convert provisioner diagnostics to errors so that users can get
context from provisioner failures.
Return diagnostics from the builtin provisioners that can be annotated
with configuration context and instance addresses.
When an attribute value changes in sensitivity, we previously rendered
this in the diff with a `~` update action and a note about the
consequence of the sensitivity change. Since we also suppress the
attribute value, this made it impossible to know if the underlying value
was changing, too, which has significant consequences on the meaning of
the plan.
This commit adds an equality check of the old/new underlying values. If
these are unchanged, we add a note to the sensitivity warning to clarify
that only sensitivity is changing.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
Once a plugin process is started, go-plugin will redirect the stdout and
stderr stream through a grpc service and provide those streams to the
client. This is rarely used, as it is prone to failing with races
because those same file descriptors are needed for the initial handshake
and logging setup, but data may be accidentally sent to these
nonetheless.
The usual culprits are stray `fmt.Print` usage where logging was
intended, or the configuration of a logger after the os.Stderr file
descriptor was replaced by go-plugin. These situations are very hard for
provider developers to debug since the data is discarded entirely.
While there may be improvements to be made in the go-plugin package to
configure this behavior, in the meantime we can add a simple monitoring
io.Writer to the streams which will surface th data as warnings in the
logs instead of writing it to `io.Discard`
This is not currently a supported interface, but we plan to release
tool(s) that consume parts of it that are more dependable later,
separately from Terraform CLI itself.
* Add helper suggestion when failed registry err
When someone has a failed registry error on init, remind them that
they should have required_providers in every module
* Give suggestion for a provider based on reqs
Suggest another provider on a registry error, from the list of
requirements we have on init. This skips the legacy lookup
process if there is a similar provider existing in requirements.
Fixes#27506
Add a new flag `-lockfile=readonly` to `terraform init`.
It would be useful to allow us to suppress dependency lockfile changes
explicitly.
The type of the `-lockfile` flag is string rather than bool, leaving
room for future extensions to other behavior variants.
The readonly mode suppresses lockfile changes, but should verify
checksums against the information already recorded. It should conflict
with the `-upgrade` flag.
Note: In the original use-case described in #27506, I would like to
suppress adding zh hashes, but a test code here suppresses adding h1
hashes because it's easy for testing.
Co-authored-by: Alisdair McDiarmid <alisdair@users.noreply.github.com>
* providers.Interface: rename ValidateDataSourceConfig to
ValidateDataResourceConfig
This PR came about after renaming ValidateResourceTypeConfig to
ValidateResourceConfig: I now understand that we'd called it the former
instead of the latter to indicate that the function wasn't necessarily
operating on a resource that actually exists. A possibly-more-accurate
renaming of both functions might then be ValidateManagedResourceConfig
and ValidateDataResourceConfig.
The next commit will update the protocol (v6 only) as well; these are in
separate commits for reviewers and will get squashed together before
merging.
* extend renaming to protov6
This is just a prototype to gather some feedback in our ongoing research
on integration testing of Terraform modules. The hope is that by having a
command integrated into Terraform itself it'll be easier for interested
module authors to give it a try, and also easier for us to iterate quickly
based on feedback without having to coordinate across multiple codebases.
Everything about this is subject to change even in future patch releases.
Since it's a CLI command rather than a configuration language feature it's
not using the language experiments mechanism, but generates a warning
similar to the one language experiments generate in order to be clear that
backward compatibility is not guaranteed.
As part of ongoing research into Terraform testing we'd like to use an
experimental feature to validate our current understanding that expressing
tests as part of the Terraform language, as opposed to in some other
language run alongside, is a good and viable way to write practical
module integration tests.
This initial experimental incarnation of that idea is implemented as a
provider, just because that's an easier extension point for research
purposes than a first-class language feature would be. Whether this would
ultimately emerge as a provider similar to this or as custom language
constructs will be a matter for future research, if this first
experiment confirms that tests written in the Terraform language are the
best direction to take.
The previous incarnation of this experiment was an externally-developed
provider apparentlymart/testing, listed on the Terraform Registry. That
helped with showing that there are some useful tests that we can write
in the Terraform language, but integrating such a provider into Terraform
will allow us to make use of it in the also-experimental "terraform test"
command, which will follow in subsequent commits, to see how this might
fit into a development workflow.
* Add support for plugin protocol v6
This PR turns on support for plugin protocol v6. A provider can
advertise itself as supporting protocol version 6 and terraform will
use the correct client.
Todo:
The "unmanaged" providers functionality does not support protocol
version, so at the moment terraform will continue to assume that
"unmanaged" providers are on protocol v5. This will require some
upstream work on go-plugin (I believe).
I would like to convert the builtin providers to use protocol v6 in a
future PR; however it is not necessary until we remove protocol v6.
* add e2e test for using both plugin protocol versions
- copied grpcwrap and made a version that returns protocol v6 provider
- copied the test provider, provider-simple, and made a version that's
using protocol v6 with the above fun
- added an e2etest
Remove the README that had old user-facing information, replacing
it with a doc.go that describes the package and points to the
plugin SDK for external consumers.
* providers.Interface: huge renamification
This commit renames a handful of functions in the providers.Interface to
match changes made in protocol v6. The following commit implements this
change across the rest of the codebase; I put this in a separate commit
for ease of reviewing and will squash these together when merging.
One noteworthy detail: protocol v6 removes the config from the
ValidateProviderConfigResponse, since it's never been used. I chose to
leave that in place in the interface until we deprecate support for
protocol v5 entirely.
Note that none of these changes impact current providers using protocol
v5; the protocol is unchanged. Only the translation layer between the
proto and terraform have changed.
It's pretty common to want to apply the various fmt.Fprint... functions
to our two output streams, and so to make that much less noisy at the
callsite here we have a small number of very thin wrappers around the
underlying fmt package functionality.
Although we're aiming to not have too much abstraction in this "terminal"
package, this seems justified in that it is only a very thin wrapper
around functionality that most Go programmers are already familiar with,
and so the risk of this causing any surprises is low and the improvement
to readability of callers seems worth it.
This is to allow convenient testing of functions that are designed to work
directly with *terminal.Streams or the individual stream objects inside.
Because the InputStream and OutputStream APIs expose directly an *os.File,
this does some extra work to set up OS-level pipes so we can capture the
output into local buffers to make test assertions against. The idea here
is to keep the tricky stuff we need for testing confined to the test
codepaths, so that the "real" codepaths don't end up needing to work
around abstractions that are otherwise unnecessary.