Functions to quote and unquote pathnames in C-style.

Following the list discussion, define two functions, quote_c_style and
unquote_c_style, to help adopting the proposed way for quoting funny
pathname letters for GNU patch.  The rule is described in:

    http://marc.theaimsgroup.com/?l=git&m=112927316408690&w=2

Currently we do not support the leading '!', but we probably should
barf upon seeing it.  Rule B4. is interpreted to require always 3
octal digits in \XYZ notation.

Signed-off-by: Junio C Hamano <junkio@cox.net>

quote.c

@@ -12,6 +12,7 @@
  *  a'b      ==> a'\''b    ==> 'a'\''b'
  *  a!b      ==> a'\!'b    ==> 'a'\!'b'
  */
+#undef EMIT
 #define EMIT(x) ( (++len < n) && (*bp++ = (x)) )
 
 size_t sq_quote_buf(char *dst, size_t n, const char *src)
@@ -51,3 +52,167 @@ char *sq_quote(const char *src)
 	return buf;
 }
 
+/*
+ * C-style name quoting.
+ *
+ * Does one of three things:
+ *
+ * (1) if outbuf and outfp are both NULL, inspect the input name and
+ *     counts the number of bytes that are needed to hold c_style
+ *     quoted version of name, counting the double quotes around
+ *     it but not terminating NUL, and returns it.  However, if name
+ *     does not need c_style quoting, it returns 0.
+ *
+ * (2) if outbuf is not NULL, it must point at a buffer large enough
+ *     to hold the c_style quoted version of name, enclosing double
+ *     quotes, and terminating NUL.  Fills outbuf with c_style quoted
+ *     version of name enclosed in double-quote pair.  Return value
+ *     is undefined.
+ *
+ * (3) if outfp is not NULL, outputs c_style quoted version of name,
+ *     but not enclosed in double-quote pair.  Return value is undefined.
+ */
+
+int quote_c_style(const char *name, char *outbuf, FILE *outfp, int no_dq)
+{
+#undef EMIT
+#define EMIT(c) \
+	(outbuf ? (*outbuf++ = (c)) : outfp ? fputc(c, outfp) : (count++))
+
+#define EMITQ() EMIT('\\')
+
+	const char *sp;
+	int ch, count = 0, needquote = 0;
+
+	if (!no_dq)
+		EMIT('"');
+	for (sp = name; (ch = *sp++); ) {
+
+		if ((ch <= ' ') || (ch == '"') ||
+		    (ch == '\\') || (ch == 0177)) {
+			needquote = 1;
+			switch (ch) {
+			case '\a': EMITQ(); ch = 'a'; break;
+			case '\b': EMITQ(); ch = 'b'; break;
+			case '\f': EMITQ(); ch = 'f'; break;
+			case '\n': EMITQ(); ch = 'n'; break;
+			case '\r': EMITQ(); ch = 'r'; break;
+			case '\t': EMITQ(); ch = 't'; break;
+			case '\v': EMITQ(); ch = 'v'; break;
+
+			case '\\': /* fallthru */
+			case '"': EMITQ(); break;
+			case ' ':
+				break;
+			default:
+				/* octal */
+				EMITQ();
+				EMIT(((ch >> 6) & 03) + '0');
+				EMIT(((ch >> 3) & 07) + '0');
+				ch = (ch & 07) + '0';
+				break;
+			}
+		}
+		EMIT(ch);
+	}
+	if (!no_dq)
+		EMIT('"');
+	if (outbuf)
+		*outbuf = 0;
+
+	return needquote ? count : 0;
+}
+
+/*
+ * C-style name unquoting.
+ *
+ * Quoted should point at the opening double quote.  Returns
+ * an allocated memory that holds unquoted name, which the caller
+ * should free when done.  Updates endp pointer to point at
+ * one past the ending double quote if given.
+ */
+
+char *unquote_c_style(const char *quoted, const char **endp)
+{
+	const char *sp;
+	char *name = NULL, *outp = NULL;
+	int count = 0, ch, ac;
+
+#undef EMIT
+#define EMIT(c) (outp ? (*outp++ = (c)) : (count++))
+
+	if (*quoted++ != '"')
+		return NULL;
+
+	while (1) {
+		/* first pass counts and allocates, second pass fills */
+		for (sp = quoted; (ch = *sp++) != '"'; ) {
+			if (ch == '\\') {
+				switch (ch = *sp++) {
+				case 'a': ch = '\a'; break;
+				case 'b': ch = '\b'; break;
+				case 'f': ch = '\f'; break;
+				case 'n': ch = '\n'; break;
+				case 'r': ch = '\r'; break;
+				case 't': ch = '\t'; break;
+				case 'v': ch = '\v'; break;
+
+				case '\\': case '"':
+					break; /* verbatim */
+
+				case '0'...'7':
+					/* octal */
+					ac = ((ch - '0') << 6);
+					if ((ch = *sp++) < '0' || '7' < ch)
+						return NULL;
+					ac |= ((ch - '0') << 3);
+					if ((ch = *sp++) < '0' || '7' < ch)
+						return NULL;
+					ac |= (ch - '0');
+					ch = ac;
+					break;
+				default:
+					return NULL; /* malformed */
+				}
+			}
+			EMIT(ch);
+		}
+
+		if (name) {
+			*outp = 0;
+			if (endp)
+				*endp = sp;
+			return name;
+		}
+		outp = name = xmalloc(count + 1);
+	}
+}
+
+void write_name_quoted(const char *prefix, const char *name,
+		       int quote, FILE *out)
+{
+	int needquote;
+
+	if (!quote) {
+	no_quote:
+		if (prefix && prefix[0])
+			fputs(prefix, out);
+		fputs(name, out);
+		return;
+	}
+
+	needquote = 0;
+	if (prefix && prefix[0])
+		needquote = quote_c_style(prefix, NULL, NULL, 0);
+	if (!needquote)
+		needquote = quote_c_style(name, NULL, NULL, 0);
+	if (needquote) {
+		fputc('"', out);
+		if (prefix && prefix[0])
+			quote_c_style(prefix, NULL, out, 1);
+		quote_c_style(name, NULL, out, 1);
+		fputc('"', out);
+	}
+	else
+		goto no_quote;
+}

quote.h

@@ -2,6 +2,7 @@
 #define QUOTE_H
 
 #include <stddef.h>
+#include <stdio.h>
 
 /* Help to copy the thing properly quoted for the shell safety.
  * any single quote is replaced with '\'', any exclamation point
@@ -27,7 +28,14 @@
  * excluding the final null regardless of the buffer size.
  */
 
-char *sq_quote(const char *src);
-size_t sq_quote_buf(char *dst, size_t n, const char *src);
+extern char *sq_quote(const char *src);
+extern size_t sq_quote_buf(char *dst, size_t n, const char *src);
+
+extern int quote_c_style(const char *name, char *outbuf, FILE *outfp,
+			 int nodq);
+extern char *unquote_c_style(const char *quoted, const char **endp);
+
+extern void write_name_quoted(const char *prefix, const char *name,
+			      int quote, FILE *out);
 
 #endif