Re: [PATCH v3] sha1-name.c: Fix handling of revisions that contain paths with brackets

Stan Hu [off-list ref] writes:

-	if (len < 4 || name[len-1] != '}')
+	if (len < 4)
 		return -1;

The original does not expect any string given after the ^{<type>}
dereferencer, like :<path>, and that is why this function returns
very early for anything when name[len-1] is not a closing brace.

We do not do that anymore, because...?

This gives me a nagging feeling that the patch is barking up a wrong
tree.  Consider what happens when a path that does not have any
funny characters, e.g. "v1.4.0^{tree}:a/b/c", is given from the top
level of the request chain (e.g. "rev-parse v1.4.0^{tree}:a/b/c")?
The caller must be feeding this function "v1.4.0^{tree}" after
finding the colon before the path "a/b/c" and setting len to point
at the colon---otherwise we won't be checking for "}" at the end
like this.

When given "master^{tree}:a/b/{c}", wouldn't the caller be doing
the same stripping to find the colon and calling this function with
len pointing at the colon before the path (i.e. "master^{tree}")?

To put it another way, it is OK if this patch wants to shift the
responsibility of finding the colon that separates the tree-ish part
and the path part from the caller to this function, but then I would
expect changes to the caller, because now the caller does not have
to find ":a/b/c" in "v1.4.0^{tree}:a/b/c" and set up the len before
calling this function.  Why can the resulting code after applying
this patch be correct without such a matching change? 

 
-	for (sp = name + len - 1; name <= sp; sp--) {
+	for (sp = name; sp < name + len; sp++) {
 		int ch = *sp;
-		if (ch == '{' && name < sp && sp[-1] == '^')
+		if (ch == '^' && sp < name + len && sp[1] == '{')
 			break;
 	}

We used to scan from the tail (as we expected that the caller gives
us a (name, len) that ends with "^{<type>}".  The updated code
instead scans from the front, looking for "^{".  I do not
particularly mind the change of strategy, as long as it is correctly
done, but I suspect the function will stay simpler if the callee is
fixed instead.

The only troublesome case is the REV^{/...} syntax.  For example,

	HEAD^{/^Git 2.0}^{tree}:t/

would want to find the commit "HEAD^{/^Git 2.0}", peel it down to a
tree object with "^{tree}" and then take its "t/" subtree.  It used
to be that the caller (get_oid_with_context_1() has a loop that
finds a colon outside "{...}" and that finds ":t/" before calling
get_oid_1()) was responsible to give us "HEAD^{/^Git 2.0}^{tree}" by
stripping ":t/", and I presume that it is still happening, but the
above loop would terminate upon seeing "^{" immediately after HEAD,
without even realizing that it has ^{tree} after it.

-	if (sp <= name)
+
+	if (sp == name + len)
 		return -1;

-	sp++; /* beginning of type name, or closing brace for empty */
-	if (starts_with(sp, "commit}"))
+	sp += 2; /* beginning of type name, or closing brace for empty */

And this comment indicates that the code did not expect to see ^{/^Git 2.0}
before ^{tree}.  

I do not quite follow.  How can this patch be correct?

Puzzled.

$ git rev-parse "master^{/^Git 2.0}^{tree}"
bc6ce29d1ec757d9d036532531a1046db4da0d96
$ ./git rev-parse "master^{/^Git 2.0}^{tree}"
master^{/^Git 2.0}^{tree}
fatal: ambiguous argument 'master^{/^Git 2.0}^{tree}': unknown revision or path not in the working tree.