cmd/go: move runtime/debug.modinfo to runtime.modinfo

It is easier to ensure that the symbol is always present
if we move it to package runtime. Avoids init-time work.
Also moves it next to buildVersion, the other similar symbol.

Setting up for "go version <binary>".

For #31624.

Change-Id: I943724469ce6992153e701257eb6f12da88c8e4e
Reviewed-on: https://go-review.googlesource.com/c/go/+/173341
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/internal/modload/build.go

@@ -247,21 +247,15 @@ func findModule(target, path string) module.Version {
 }
 
 func ModInfoProg(info string) []byte {
-	// Inject a variable with the debug information as runtime/debug.modinfo,
+	// Inject a variable with the debug information as runtime.modinfo,
 	// but compile it in package main so that it is specific to the binary.
-	//
 	// The variable must be a literal so that it will have the correct value
 	// before the initializer for package main runs.
 	//
-	// We also want the value to be present even if runtime/debug.modinfo is
-	// otherwise unused in the rest of the program. Reading it in an init function
-	// suffices for now.
-
+	// The runtime startup code refers to the variable, which keeps it live in all binaries.
 	return []byte(fmt.Sprintf(`package main
 import _ "unsafe"
-//go:linkname __debug_modinfo__ runtime/debug.modinfo
+//go:linkname __debug_modinfo__ runtime.modinfo
 var __debug_modinfo__ = %q
-var keepalive_modinfo = __debug_modinfo__
-func init() { keepalive_modinfo = __debug_modinfo__ }
 	`, string(infoStart)+info+string(infoEnd)))
 }

src/runtime/debug.go

@@ -57,3 +57,8 @@ func NumCgoCall() int64 {
 func NumGoroutine() int {
 	return int(gcount())
 }
+
+//go:linkname debug_modinfo runtime/debug.modinfo
+func debug_modinfo() string {
+	return modinfo
+}

src/runtime/debug/mod.go

@@ -8,14 +8,14 @@ import (
 	"strings"
 )
 
-// set using cmd/go/internal/modload.ModInfoProg
-var modinfo string
+// exported from runtime
+func modinfo() string
 
 // ReadBuildInfo returns the build information embedded
 // in the running binary. The information is available only
 // in binaries built with module support.
 func ReadBuildInfo() (info *BuildInfo, ok bool) {
-	return readBuildInfo(modinfo)
+	return readBuildInfo(modinfo())
 }
 
 // BuildInfo represents the build information read from

src/runtime/proc.go

@@ -13,6 +13,9 @@ import (
 
 var buildVersion = sys.TheVersion
 
+// set using cmd/go/internal/modload.ModInfoProg
+var modinfo string
+
 // Goroutine scheduler
 // The scheduler's job is to distribute ready-to-run goroutines over worker threads.
 //
@@ -577,6 +580,11 @@ func schedinit() {
 		// to ensure runtime·buildVersion is kept in the resulting binary.
 		buildVersion = "unknown"
 	}
+	if len(modinfo) == 1 {
+		// Condition should never trigger. This code just serves
+		// to ensure runtime·modinfo is kept in the resulting binary.
+		modinfo = ""
+	}
 }
 
 func dumpgstatus(gp *g) {