i18n: handle write errors in xgettext-go

The old code was too naive and would ignore write errors in
writePotFile silently. This PR fixes this and makes it fail
explicitly if anything with the write goes wrong.

This may help with LP#1758684 in which the snappy.pot file
is truncated.

i18n/xgettext-go/main.go

@@ -172,13 +172,13 @@ func processFiles(args []string) error {
 func processSingleGoSource(fset *token.FileSet, fname string) error {
 	fnameContent, err := ioutil.ReadFile(fname)
 	if err != nil {
-		panic(err)
+		return err
 	}
 
 	// Create the AST by parsing src.
 	f, err := parser.ParseFile(fset, fname, fnameContent, parser.ParseComments)
 	if err != nil {
-		panic(err)
+		return err
 	}
 
 	ast.Inspect(f, func(n ast.Node) bool {
@@ -192,6 +192,14 @@ var formatTime = func() string {
 	return time.Now().Format("2006-01-02 15:04-0700")
 }
 
+func mustFprintf(w io.Writer, format string, a ...interface{}) (n int, err error) {
+	n, err = fmt.Fprintf(w, format, a...)
+	if err != nil {
+		panic(fmt.Sprintf("cannot write output: %v", err))
+	}
+	return n, err
+}
+
 func writePotFile(out io.Writer) {
 
 	header := fmt.Sprintf(`# SOME DESCRIPTIVE TITLE.
@@ -213,7 +221,7 @@ msgstr  "Project-Id-Version: %s\n"
         "Content-Transfer-Encoding: 8bit\n"
 
 `, opts.PackageName, opts.MsgIDBugsAddress, formatTime())
-	fmt.Fprintf(out, "%s", header)
+	mustFprintf(out, "%s", header)
 
 	// yes, this is the way to do it in go
 	sortedKeys := []string{}
@@ -229,19 +237,19 @@ msgstr  "Project-Id-Version: %s\n"
 		msgidList := msgIDs[k]
 		for _, msgid := range msgidList {
 			if opts.AddComments || opts.AddCommentsTag != "" {
-				fmt.Fprintf(out, "%s", msgid.comment)
+				mustFprintf(out, "%s", msgid.comment)
 			}
 		}
 		if !opts.NoLocation {
-			fmt.Fprintf(out, "#:")
+			mustFprintf(out, "#:")
 			for _, msgid := range msgidList {
-				fmt.Fprintf(out, " %s:%d", msgid.fname, msgid.line)
+				mustFprintf(out, " %s:%d", msgid.fname, msgid.line)
 			}
-			fmt.Fprintf(out, "\n")
+			mustFprintf(out, "\n")
 		}
 		msgid := msgidList[0]
 		if msgid.formatHint != "" {
-			fmt.Fprintf(out, "#, %s\n", msgid.formatHint)
+			mustFprintf(out, "#, %s\n", msgid.formatHint)
 		}
 		var formatOutput = func(in string) string {
 			// split string with \n into multiple lines
@@ -250,15 +258,15 @@ msgstr  "Project-Id-Version: %s\n"
 			// cleanup too aggressive splitting (empty "" lines)
 			return strings.TrimSuffix(out, "\"\n        \"")
 		}
-		fmt.Fprintf(out, "msgid   \"%v\"\n", formatOutput(k))
+		mustFprintf(out, "msgid   \"%v\"\n", formatOutput(k))
 		if msgid.msgidPlural != "" {
-			fmt.Fprintf(out, "msgid_plural   \"%v\"\n", formatOutput(msgid.msgidPlural))
-			fmt.Fprintf(out, "msgstr[0]  \"\"\n")
-			fmt.Fprintf(out, "msgstr[1]  \"\"\n")
+			mustFprintf(out, "msgid_plural   \"%v\"\n", formatOutput(msgid.msgidPlural))
+			mustFprintf(out, "msgstr[0]  \"\"\n")
+			mustFprintf(out, "msgstr[1]  \"\"\n")
 		} else {
-			fmt.Fprintf(out, "msgstr  \"\"\n")
+			mustFprintf(out, "msgstr  \"\"\n")
 		}
-		fmt.Fprintf(out, "\n")
+		mustFprintf(out, "\n")
 	}
 
 }