tree fb495c96e69a1e688d26687e29e688886ffe56b1
parent 9fe99e262126467c4cb2f44cf941767d29fb33d8
author David 'Digit' Turner <digit@google.com> 1790777168 +0200
committer gn-scoped@luci-project-accounts.iam.gserviceaccount.com <gn-scoped@luci-project-accounts.iam.gserviceaccount.com> 1790943590 -0700

Delayed generated_file() writes if they collect validations metadata

When a generated_file() target is written to the Ninja build
plan, delay its output file write if `collect_validations_metadata`
is true. Instead, the writes will be performed after all targets
have been resolved to ensure there are no runtime crashes when
performing the metadata walk.

Note that the flag can be set directly in the target
definition, or inherited from the .gn default value set
through experimental_collect_validations_metadata.

- builder_record.h: Add a comment explaining why metadata
  walks that include validations must be delayed.

- ninja_generated_file_target_writer.cc: If a target
  has collect_validations_metadata enabled, do not write
  its output file yet, instead set a boolean flag
  to indicate the delay.

- ninja_target_writer.cc: Take an optional pointer to
  a boolean flag to receive notification of delayed
  writes for generated_file() targets. Pass the pointer
  to NinjaGeneratedFileWriter properly.

- setup.cc: Add SetPostResolutionCallback() to register
  a callback to invoke after all items have been resolved
  in the graph (i.e. after the first scheduler.Run() call).

- setup_unittest.cc: Add new unit-test that verify that
  metadata walks with validations do not crash and return
  the correct values (or errors in case of missing deps).

- command_gen.cc: Record delayed generated_file() target
  pointers during graph resolution, then use a post-resolution
  callback to write all delayed generated_file() outputs
  in parallel.

Benchmarking shows that:

- For a medium Fuchsia project (which currently relies on
  validations metadata walks), using
  `gn gen --experimental-collect-validations-metadata` is
  about 15% slower with this patch applied (18.55s to
  20.3s)

- For a default Android/arm64 Chromium build,
  `gn gen --experimental-collect-validations-metadata`
  there is no statical difference (around 3s each).

Bug: 566346002
Change-Id: I58ad0c3bcec71ceec80cf5a617ef00e86415734d
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/27086
Reviewed-by: Takuto Ikuta <tikuta@google.com>
Reviewed-by: Matt Stark <msta@google.com>
Commit-Queue: David Turner <digit@google.com>