runtime: add hook to register coverage-instrumented packages

Add support to the runtime for registering coverage-instrumented
packages, using a new hook that can be called from the init function
of an instrumented package. The hook records the meta-data symbol for
the package (chaining it onto a list), and returns a package ID to be
used to identify functions in the package. This new hook is not yet
called; that will be added in a subsequent patch. The list of
registered meta-data objects will be used (again in a future patch) as
part of coverage data file writing.

Special handling is required for packages such as "runtime" or
"internal/cpu", where functions in the package execute before the
package "init" func runs. For these packages hard-code the package ID,
then record the position of the package in the overall list so that we
can fix things up later on.

Updates #51430.

Change-Id: I6ca3ddf535197442a2603c6d7a0a9798b8496f40
Reviewed-on: https://go-review.googlesource.com/c/go/+/401234
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Than McIntosh <thanm@google.com>

src/internal/coverage/pkid.go

@@ -0,0 +1,80 @@
+// 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
+
+// Building the runtime package with coverage instrumentation enabled
+// is tricky.  For all other packages, you can be guaranteed that
+// the package init function is run before any functions are executed,
+// but this invariant is not maintained for packages such as "runtime",
+// "internal/cpu", etc. To handle this, hard-code the package ID for
+// the set of packages whose functions may be running before the
+// init function of the package is complete.
+//
+// Hardcoding is unfortunate because it means that the tool that does
+// coverage instrumentation has to keep a list of runtime packages,
+// meaning that if someone makes changes to the pkg "runtime"
+// dependencies, unexpected behavior will result for coverage builds.
+// The coverage runtime will detect and report the unexpected
+// behavior; look for an error of this form:
+//
+//    internal error in coverage meta-data tracking:
+//    list of hard-coded runtime package IDs needs revising.
+//    registered list:
+//    slot: 0 path='internal/cpu'  hard-coded id: 1
+//    slot: 1 path='internal/goarch'  hard-coded id: 2
+//    slot: 2 path='runtime/internal/atomic'  hard-coded id: 3
+//    slot: 3 path='internal/goos'
+//    slot: 4 path='runtime/internal/sys'  hard-coded id: 5
+//    slot: 5 path='internal/abi'  hard-coded id: 4
+//    slot: 6 path='runtime/internal/math'  hard-coded id: 6
+//    slot: 7 path='internal/bytealg'  hard-coded id: 7
+//    slot: 8 path='internal/goexperiment'
+//    slot: 9 path='runtime/internal/syscall'  hard-coded id: 8
+//    slot: 10 path='runtime'  hard-coded id: 9
+//    fatal error: runtime.addCovMeta
+//
+// For the error above, the hard-coded list is missing "internal/goos"
+// and "internal/goexperiment" ; the developer in question will need
+// to copy the list above into "rtPkgs" below.
+//
+// Note: this strategy assumes that the list of dependencies of
+// package runtime is fixed, and doesn't vary depending on OS/arch. If
+// this were to be the case, we would need a table of some sort below
+// as opposed to a fixed list.
+
+var rtPkgs = [...]string{
+	"internal/cpu",
+	"internal/goarch",
+	"runtime/internal/atomic",
+	"internal/goos",
+	"runtime/internal/sys",
+	"internal/abi",
+	"runtime/internal/math",
+	"internal/bytealg",
+	"internal/goexperiment",
+	"runtime/internal/syscall",
+	"runtime",
+}
+
+// Scoping note: the constants and apis in this file are internal
+// only, not expected to ever be exposed outside of the runtime (unlike
+// other coverage file formats and APIs, which will likely be shared
+// at some point).
+
+// NotHardCoded is a package pseudo-ID indicating that a given package
+// is not part of the runtime and doesn't require a hard-coded ID.
+const NotHardCoded = -1
+
+// HardCodedPkgId returns the hard-coded ID for the specified package
+// path, or -1 if we don't use a hard-coded ID. Hard-coded IDs start
+// at -2 and decrease as we go down the list.
+func HardCodedPkgID(pkgpath string) int {
+	for k, p := range rtPkgs {
+		if p == pkgpath {
+			return (0 - k) - 2
+		}
+	}
+	return NotHardCoded
+}

src/internal/coverage/rtcov/rtcov.go

@@ -0,0 +1,25 @@
+// 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 rtcov
+
+// This package contains types whose structure is shared between
+// the runtime package and the "runtime/coverage" package.
+
+// CovMetaBlob is a container for holding the meta-data symbol (an
+// RODATA variable) for an instrumented Go package. Here "p" points to
+// the symbol itself, "len" is the length of the sym in bytes, and
+// "hash" is an md5sum for the sym computed by the compiler. When
+// the init function for a coverage-instrumented package executes, it
+// will make a call into the runtime which will create a covMetaBlob
+// object for the package and chain it onto a global list.
+type CovMetaBlob struct {
+	P                  *byte
+	Len                uint32
+	Hash               [16]byte
+	PkgPath            string
+	PkgID              int
+	CounterMode        uint8 // coverage.CounterMode
+	CounterGranularity uint8 // coverage.CounterGranularity
+}