cmd/link: use current GOROOT for source file paths for standard library

This CL changes the source file information in the
standard library's .a files to say "$GOROOT/src/runtime/chan.go"
(with a literal "$GOROOT") instead of spelling out the actual directory.
The linker then substitutes the actual $GOROOT (or $GOROOT_FINAL)
as appropriate.

If people download a binary distribution to an alternate location,
following the instructions at https://golang.org/doc/install#install,
the code before this CL would end up with source paths pointing to
/usr/local/go no matter where the actual sources were.
Now the source paths for built binaries will point to the actual sources
(hopefully).

The source line information in distributed binaries is not affected:
those will still say /usr/local/go. But binaries people build themselves
(their own programs, not the go distribution programs) will be correct.

Fixing this path also fixes the lookup of the runtime-gdb.py file.

Fixes #5533.

Change-Id: I03729baae3fbd8cd636e016275ee5ad2606e4663
Reviewed-on: https://go-review.googlesource.com/18200
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/obj/obj.go

@@ -74,15 +74,15 @@ func (h *LineHist) setFile(stk *LineStack, file string) {
 		abs = filepath.Join(h.Dir, file)
 	}
 
-	// Remove leading TrimPathPrefix, or else rewrite $GOROOT to $GOROOT_FINAL.
+	// Remove leading TrimPathPrefix, or else rewrite $GOROOT to literal $GOROOT.
 	if h.TrimPathPrefix != "" && hasPathPrefix(abs, h.TrimPathPrefix) {
 		if abs == h.TrimPathPrefix {
 			abs = ""
 		} else {
 			abs = abs[len(h.TrimPathPrefix)+1:]
 		}
-	} else if h.GOROOT_FINAL != "" && h.GOROOT_FINAL != h.GOROOT && hasPathPrefix(abs, h.GOROOT) {
+	} else if hasPathPrefix(abs, h.GOROOT) {
-		abs = h.GOROOT_FINAL + abs[len(h.GOROOT):]
+		abs = "$GOROOT" + abs[len(h.GOROOT):]
 	}
 	if abs == "" {
 		abs = "??"

src/cmd/link/internal/ld/pcln.go

@@ -8,6 +8,8 @@ import (
 	"cmd/internal/obj"
 	"fmt"
 	"log"
+	"os"
+	"path/filepath"
 )
 
 // funcpctab writes to dst a pc-value table mapping the code in func to the values
@@ -150,6 +152,7 @@ func renumberfiles(ctxt *Link, files []*LSym, d *Pcdata) {
 			f.Value = int64(ctxt.Nhistfile)
 			f.Type = obj.SFILEPATH
 			f.Next = ctxt.Filesyms
+			f.Name = expandGoroot(f.Name)
 			ctxt.Filesyms = f
 		}
 	}
@@ -376,6 +379,18 @@ func pclntab() {
 	}
 }
 
+func expandGoroot(s string) string {
+	const n = len("$GOROOT")
+	if len(s) >= n+1 && s[:n] == "$GOROOT" && (s[n] == '/' || s[n] == '\\') {
+		root := goroot
+		if final := os.Getenv("GOROOT_FINAL"); final != "" {
+			root = final
+		}
+		return filepath.ToSlash(filepath.Join(root, s[n:]))
+	}
+	return s
+}
+
 const (
 	BUCKETSIZE    = 256 * MINFUNC
 	SUBBUCKETS    = 16

src/runtime/runtime-gdb_test.go

@@ -87,6 +87,7 @@ func TestGdbPython(t *testing.T) {
 
 	args := []string{"-nx", "-q", "--batch", "-iex",
 		fmt.Sprintf("add-auto-load-safe-path %s/src/runtime", runtime.GOROOT()),
+		"-ex", "info auto-load python-scripts",
 		"-ex", "br main.go:10",
 		"-ex", "run",
 		"-ex", "echo BEGIN info goroutines\n",
@@ -129,7 +130,10 @@ func TestGdbPython(t *testing.T) {
 			t.Skipf("skipping because GOROOT=%s does not exist", runtime.GOROOT())
 		}
 
-		t.Fatalf("failed to load Go runtime support: %s", firstLine)
+		_, file, _, _ := runtime.Caller(1)
+
+		t.Logf("package testing source file: %s", file)
+		t.Fatalf("failed to load Go runtime support: %s\n%s", firstLine, got)
 	}
 
 	// Extract named BEGIN...END blocks from output