runtime/coverage: runtime routines to emit coverage data

This patch fleshes out the runtime support for emitting coverage data
at the end of a run of an instrumented binary. Data is emitted in the
form of a pair of files, a meta-out-file and counter-data-outfile,
each written to the dir GOCOVERDIR. The meta-out-file is emitted only
if required; no need to emit again if an existing meta-data file with
the same hash and length is present.

Updates #51430.

Change-Id: I59d20a4b8c05910c933ee29527972f8e401b1685
Reviewed-on: https://go-review.googlesource.com/c/go/+/355451
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Than McIntosh <thanm@google.com>

1 files changed, 42 insertions, 0 deletions

diff --git a/src/runtime/coverage/hooks.go b/src/runtime/coverage/hooks.go
new file mode 100644
index 0000000000..a9fbf9d7dd
--- /dev/null
+++ b/src/runtime/coverage/hooks.go
@@ -0,0 +1,42 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package coverage
+
+import _ "unsafe"
+
+// initHook is invoked from the main package "init" routine in
+// programs built with "-cover". This function is intended to be
+// called only by the compiler.
+//
+// If 'istest' is false, it indicates we're building a regular program
+// ("go build -cover ..."), in which case we immediately try to write
+// out the meta-data file, and register emitCounterData as an exit
+// hook.
+//
+// If 'istest' is true (indicating that the program in question is a
+// Go test binary), then we tentatively queue up both emitMetaData and
+// emitCounterData as exit hooks. In the normal case (e.g. regular "go
+// test -cover" run) the testmain.go boilerplate will run at the end
+// of the test, write out the coverage percentage, and then invoke
+// markProfileEmitted() to indicate that no more work needs to be
+// done. If however that call is never made, this is a sign that the
+// test binary is being used as a replacement binary for the tool
+// being tested, hence we do want to run exit hooks when the program
+// terminates.
+func initHook(istest bool) {
+	// Note: hooks are run in reverse registration order, so
+	// register the counter data hook before the meta-data hook
+	// (in the case where two hooks are needed).
+	runOnNonZeroExit := true
+	runtime_addExitHook(emitCounterData, runOnNonZeroExit)
+	if istest {
+		runtime_addExitHook(emitMetaData, runOnNonZeroExit)
+	} else {
+		emitMetaData()
+	}
+}
+
+//go:linkname runtime_addExitHook runtime.addExitHook
+func runtime_addExitHook(f func(), runOnNonZeroExit bool)