[PATCH v3 1/2] abspath: add a function to resolve paths with missing components

["brian m. carlson" <sandals@xxxxxxxxxxxxxxxxxxxx>]

We'd like to canonicalize paths such that we can preserve any number of
trailing components that may be missing.  Let's add a function to do
that, taking the number of components to canonicalize, and make
strbuf_realpath a wrapper around it that allows just one component.

Signed-off-by: brian m. carlson <sandals@xxxxxxxxxxxxxxxxxxxx>
---
 abspath.c | 33 ++++++++++++++++++++++++++++++++-
 cache.h   |  2 ++
 2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/abspath.c b/abspath.c
index 6f15a418bb..1d8f3d007c 100644
--- a/abspath.c
+++ b/abspath.c
@@ -20,6 +20,7 @@ static void strip_last_component(struct strbuf *path)
 	/* Find start of the last component */
 	while (offset < len && !is_dir_sep(path->buf[len - 1]))
 		len--;
+
 	/* Skip sequences of multiple path-separators */
 	while (offset < len && is_dir_sep(path->buf[len - 1]))
 		len--;
@@ -66,6 +67,22 @@ static void get_root_part(struct strbuf *resolved, struct strbuf *remaining)
 #define MAXSYMLINKS 32
 #endif
 
+/* Count non-contiguous directory separators, not including a trailing one. */
+static int count_dir_separators(const char *s)
+{
+	int count = 0;
+	int last_sep = 0;
+	const char *p = s;
+	while (*p) {
+		int is_sep = is_dir_sep(*p++);
+		if (is_sep && !last_sep)
+			count++;
+		last_sep = is_sep;
+	}
+	return count;
+}
+
+
 /*
  * Return the real path (i.e., absolute path, with symlinks resolved
  * and extra slashes removed) equivalent to the specified path.  (If
@@ -80,6 +97,16 @@ static void get_root_part(struct strbuf *resolved, struct strbuf *remaining)
  */
 char *strbuf_realpath(struct strbuf *resolved, const char *path,
 		      int die_on_error)
+{
+	return strbuf_realpath_missing(resolved, path, 1, die_on_error);
+}
+
+/*
+ * Just like strbuf_realpath, but allows specifying how many missing components
+ * are permitted.  -1 may be specified to allow an unlimited number.
+ */
+char *strbuf_realpath_missing(struct strbuf *resolved, const char *path,
+			      int missing_components, int die_on_error)
 {
 	struct strbuf remaining = STRBUF_INIT;
 	struct strbuf next = STRBUF_INIT;
@@ -128,8 +155,12 @@ char *strbuf_realpath(struct strbuf *resolved, const char *path,
 		strbuf_addbuf(resolved, &next);
 
 		if (lstat(resolved->buf, &st)) {
+			int trailing_components = count_dir_separators(remaining.buf) +
+						  (remaining.len != 0);
 			/* error out unless this was the last component */
-			if (errno != ENOENT || remaining.len) {
+			if (errno != ENOENT ||
+			    !(missing_components == -1 ||
+			      trailing_components < missing_components)) {
 				if (die_on_error)
 					die_errno("Invalid path '%s'",
 						  resolved->buf);
diff --git a/cache.h b/cache.h
index c0072d43b1..ee4bc5ec04 100644
--- a/cache.h
+++ b/cache.h
@@ -1320,6 +1320,8 @@ static inline int is_absolute_path(const char *path)
 int is_directory(const char *);
 char *strbuf_realpath(struct strbuf *resolved, const char *path,
 		      int die_on_error);
+char *strbuf_realpath_missing(struct strbuf *resolved, const char *path,
+			      int missing_components, int die_on_error);
 char *real_pathdup(const char *path, int die_on_error);
 const char *absolute_path(const char *path);
 char *absolute_pathdup(const char *path);