cmd/dist: prevent race on VERSION creation

Commands such as "dist version > VERSION" will cause
the shell to create an empty VERSION file and set dist's
stdout to its fd. dist in turn looks at VERSION and uses
its content if available, which is empty at this point.

Fix that by ignoring VERSION if it's empty.

Also prevent cmdversion from running findgoversion a
second time. It was already loaded by init.

R=adg, gustavo, rsc
CC=golang-dev
https://golang.org/cl/5639044

1 files changed, 7 insertions, 2 deletions

diff --git a/src/cmd/dist/build.c b/src/cmd/dist/build.c
index 54510db1da..d5cf17dcd3 100644
--- a/src/cmd/dist/build.c
+++ b/src/cmd/dist/build.c
@@ -179,7 +179,12 @@ findgoversion(void)
 	if(isfile(bstr(&path))) {
 		readfile(&b, bstr(&path));
 		chomp(&b);
-		goto done;
+		// Commands such as "dist version > VERSION" will cause
+		// the shell to create an empty VERSION file and set dist's
+		// stdout to its fd. dist in turn looks at VERSION and uses
+		// its content if available, which is empty at this point.
+		if(b.len > 0)
+			goto done;
 	}
 
 	// The $GOROOT/VERSION.cache file is a cache to avoid invoking
@@ -1370,5 +1375,5 @@ cmdversion(int argc, char **argv)
 	if(argc > 0)
 		usage();
 
-	xprintf("%s\n", findgoversion());
+	xprintf("%s\n", goversion);
 }