The config callback functions get keys of the general form:

  section.subsection.key

(where the subsection may be contain arbitrary data, or may
be missing). For matching keys without subsections, it is
simple enough to call "strcmp". Matching keys with
subsections is a little more complicated, and each callback
does it in an ad-hoc way, usually involving error-prone
pointer arithmetic.

Let's provide a helper that keeps the pointer arithmetic all
in one place.

Signed-off-by: Jeff King <p...@peff.net>
---
No users yet; they come in future patches.

 cache.h  | 15 +++++++++++++++
 config.c | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/cache.h b/cache.h
index c257953..14003b8 100644
--- a/cache.h
+++ b/cache.h
@@ -1164,6 +1164,21 @@ extern int git_config_include(const char *name, const 
char *value, void *data);
 #define CONFIG_INCLUDE_INIT { 0 }
 extern int git_config_include(const char *name, const char *value, void *data);
 
+/*
+ * Match and parse a config key of the form:
+ *
+ *   section.(subsection.)?key
+ *
+ * (i.e., what gets handed to a config_fn_t). The caller provides the section;
+ * we return -1 if it does not match, 0 otherwise. The subsection and key
+ * out-parameters are filled by the function (and subsection is NULL if it is
+ * missing).
+ */
+extern int match_config_key(const char *var,
+                    const char *section,
+                    const char **subsection, int *subsection_len,
+                    const char **key);
+
 extern int committer_ident_sufficiently_given(void);
 extern int author_ident_sufficiently_given(void);
 
diff --git a/config.c b/config.c
index 7b444b6..18d9c0e 100644
--- a/config.c
+++ b/config.c
@@ -1667,3 +1667,36 @@ int config_error_nonbool(const char *var)
 {
        return error("Missing value for '%s'", var);
 }
+
+int match_config_key(const char *var,
+                    const char *section,
+                    const char **subsection, int *subsection_len,
+                    const char **key)
+{
+       int section_len = strlen(section);
+       const char *dot;
+
+       /* Does it start with "section." ? */
+       if (prefixcmp(var, section) || var[section_len] != '.')
+               return -1;
+
+       /*
+        * Find the key; we don't know yet if we have a subsection, but we must
+        * parse backwards from the end, since the subsection may have dots in
+        * it, too.
+        */
+       dot = strrchr(var, '.');
+       *key = dot + 1;
+
+       /* Did we have a subsection at all? */
+       if (dot == var + section_len) {
+               *subsection = NULL;
+               *subsection_len = 0;
+       }
+       else {
+               *subsection = var + section_len + 1;
+               *subsection_len = dot - *subsection;
+       }
+
+       return 0;
+}
-- 
1.8.1.rc1.10.g7d71f7b

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