strbuf: add strbuf_getcwd()

Add strbuf_getcwd(), which puts the current working directory into a
strbuf.  Because it doesn't use a fixed-size buffer it supports
arbitrarily long paths, provided the platform's getcwd() does as well.
At least on Linux and FreeBSD it handles paths longer than PATH_MAX
just fine.

Suggested-by: Karsten Blees <karsten.blees@gmail.com>
Helped-by: Duy Nguyen <pclouds@gmail.com>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-strbuf.txt

@@ -289,6 +289,10 @@ same behaviour as well.
 	use it unless you need the correct position in the file
 	descriptor.
 
+`strbuf_getcwd`::
+
+	Set the buffer to the path of the current working directory.
+
 `stripspace`::
 
 	Strip whitespace from a buffer. The second parameter controls if

strbuf.c

@@ -398,6 +398,27 @@ int strbuf_readlink(struct strbuf *sb, const char *path, size_t hint)
 	return -1;
 }
 
+int strbuf_getcwd(struct strbuf *sb)
+{
+	size_t oldalloc = sb->alloc;
+	size_t guessed_len = 128;
+
+	for (;; guessed_len *= 2) {
+		strbuf_grow(sb, guessed_len);
+		if (getcwd(sb->buf, sb->alloc)) {
+			strbuf_setlen(sb, strlen(sb->buf));
+			return 0;
+		}
+		if (errno != ERANGE)
+			break;
+	}
+	if (oldalloc == 0)
+		strbuf_release(sb);
+	else
+		strbuf_reset(sb);
+	return -1;
+}
+
 int strbuf_getwholeline(struct strbuf *sb, FILE *fp, int term)
 {
 	int ch;

strbuf.h

@@ -163,6 +163,7 @@ extern size_t strbuf_fread(struct strbuf *, size_t, FILE *);
 extern ssize_t strbuf_read(struct strbuf *, int fd, size_t hint);
 extern int strbuf_read_file(struct strbuf *sb, const char *path, size_t hint);
 extern int strbuf_readlink(struct strbuf *sb, const char *path, size_t hint);
+extern int strbuf_getcwd(struct strbuf *sb);
 
 extern int strbuf_getwholeline(struct strbuf *, FILE *, int);
 extern int strbuf_getline(struct strbuf *, FILE *, int);