commit 8404fb4c7b6cb698cdc95a2e360e19341fdc365e
author John Millikin <john@john-millikin.com> Tue Nov 05 09:39:40 2019 -0800
committer Copybara-Service <copybara-worker@google.com> Tue Nov 05 09:40:51 2019 -0800
tree 9e3582a637afa2b758eeb70c6f31d236aa8d4ae0
parent 1199813a2353629b5a7605ac4b6ac54319cfce1d

Avoid breaking toolchain resolution if platform has defaulted constraint.

### Summary
See https://github.com/bazelbuild/bazel/issues/8778 and https://github.com/bazelbuild/bazel/issues/8274 for context / motivation. In summary, the presence of a `constraint_setting` with `default_constraint_value` in a Platform's constraints will cause toolchain resolution to break for all toolchains that do not explicitly set that constraint value. This makes `default_constraint_value` dangerous because setting it can break other rulesets composed into the same top-level workspace.

cc @jayconrod @m3rcuriel @katre

### Test Plan
I updated the unit test `SingleToolchainResolutionFunction`, verified it failed, then altered the implementation to pass. I also built `//src:bazel` and ran it with a test workspace to verify the end result worked as expected.

Test workspace:
```
$ cat WORKSPACE
register_toolchains("//:my_toolchain")
$ cat BUILD
load(":rules.bzl", "my_toolchain_info", "my_rule")

my_rule(name = "hello")
toolchain_type(name = "my_toolchain_type")
my_toolchain_info(name = "my_toolchain_info")
toolchain(
    name = "my_toolchain",
    toolchain = "//:my_toolchain_info",
    toolchain_type = "//:my_toolchain_type",
)

constraint_setting(
    name = "setting",
    default_constraint_value = ":value_a",
)
constraint_value(
    name = "value_a",
    constraint_setting = ":setting",
)
constraint_value(
    name = "value_b",
    constraint_setting = ":setting",
)
platform(
    name = "platform_a",
    constraint_values = ["//:value_a"],
)
platform(
    name = "platform_b",
    constraint_values = ["//:value_b"],
)

$ cat rules.bzl
def _my_toolchain_info(ctx):
    return platform_common.ToolchainInfo()

my_toolchain_info = rule(_my_toolchain_info)

def _my_rule(ctx):
    pass

my_rule = rule(_my_rule, toolchains = ["//:my_toolchain_type"])
```

Before:
```
$ bazel build --toolchain_resolution_debug --host_platform=//:platform_b //:hello
[...]
INFO: ToolchainResolution: Looking for toolchain of type //:my_toolchain_type...
INFO: ToolchainResolution:   Considering toolchain //:my_toolchain_info...
INFO: ToolchainResolution:     Toolchain constraint //:setting has value //:value_a, which does not match value //:value_b from the target platform //:platform_b
INFO: ToolchainResolution:   Rejected toolchain //:my_toolchain_info, because of target platform mismatch
INFO: ToolchainResolution:   No toolchains found
ERROR: While resolving toolchains for target //:hello: no matching toolchains found for types //:my_toolchain_type
```

After:
```
$ bazel build --toolchain_resolution_debug --host_platform=//:platform_b //:hello
[...]
INFO: ToolchainResolution: Looking for toolchain of type //:my_toolchain_type...
INFO: ToolchainResolution:   Considering toolchain //:my_toolchain_info...
INFO: ToolchainResolution:   For toolchain type //:my_toolchain_type, possible execution platforms and toolchains: {//:platform_b -> //:my_toolchain_info}
INFO: ToolchainResolution: Selected execution platform //:platform_b, type //:my_toolchain_type -> toolchain //:my_toolchain_info
INFO: ToolchainResolution: Selected execution platform //:platform_b,
INFO: Analyzed target //:hello (9 packages loaded, 27 targets configured).
[...]
```

Closes #10102.

PiperOrigin-RevId: 278645171