On Sun, Jul 13, 2014 at 01:33:56PM -0400, Jeff King wrote:

> I realize that I am reinventing the error-reporting wheel on a sleepy
> Sunday afternoon without having thought about it much, so there is
> probably some gotcha or case that makes this ugly, or perhaps it just
> ends up verbose in practice. But one can dream.

Just for fun...

---
diff --git a/builtin/tag.c b/builtin/tag.c
index 5e0744b..0f5be3b 100644
--- a/builtin/tag.c
+++ b/builtin/tag.c
@@ -379,7 +379,10 @@ static int git_tag_config(const char *var, const char 
*value, void *cb)
        if (!strcmp(var, "tag.sort")) {
                if (!value)
                        return config_error_nonbool(var);
-               parse_sort_string(value, &tag_sort);
+               set_error_routine(collect_errors);
+               if (parse_sort_string(value, &tag_sort) < 0)
+                       print_errors(warning, "#{error} in config option 
'tag.sort'");
+               pop_error_routine();
                return 0;
        }
 
diff --git a/git-compat-util.h b/git-compat-util.h
index 9de3180..6bf91a6 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -346,6 +346,11 @@ extern void set_die_routine(NORETURN_PTR void 
(*routine)(const char *err, va_lis
 extern void set_error_routine(void (*routine)(const char *err, va_list 
params));
 extern void set_die_is_recursing_routine(int (*routine)(void));
 
+void pop_error_routine(void);
+void collect_errors(const char *err, va_list params);
+__attribute__((format (printf, 2, 3)))
+void print_errors(void (*func)(const char *, ...), const char *fmt, ...);
+
 extern int starts_with(const char *str, const char *prefix);
 extern int ends_with(const char *str, const char *suffix);
 
diff --git a/usage.c b/usage.c
index ed14645..055ccc7 100644
--- a/usage.c
+++ b/usage.c
@@ -57,10 +57,16 @@ static int die_is_recursing_builtin(void)
  * (ugh), so keep things static. */
 static NORETURN_PTR void (*usage_routine)(const char *err, va_list params) = 
usage_builtin;
 static NORETURN_PTR void (*die_routine)(const char *err, va_list params) = 
die_builtin;
-static void (*error_routine)(const char *err, va_list params) = error_builtin;
 static void (*warn_routine)(const char *err, va_list params) = warn_builtin;
 static int (*die_is_recursing)(void) = die_is_recursing_builtin;
 
+struct error_func_list {
+       void (*func)(const char *, va_list);
+       struct error_func_list *next;
+};
+static struct error_func_list default_error_func = { error_builtin };
+static struct error_func_list *error_funcs = &default_error_func;
+
 void set_die_routine(NORETURN_PTR void (*routine)(const char *err, va_list 
params))
 {
        die_routine = routine;
@@ -68,7 +74,84 @@ void set_die_routine(NORETURN_PTR void (*routine)(const char 
*err, va_list param
 
 void set_error_routine(void (*routine)(const char *err, va_list params))
 {
-       error_routine = routine;
+       struct error_func_list *efl = xmalloc(sizeof(*efl));
+       efl->func = routine;
+       efl->next = error_funcs;
+       error_funcs = efl;
+}
+
+void pop_error_routine(void)
+{
+       while (error_funcs != &default_error_func) {
+               struct error_func_list *efl = error_funcs;
+               error_funcs = efl->next;
+               free(efl);
+       }
+}
+
+struct error_list {
+       struct strbuf buf;
+       struct error_list *next;
+};
+struct error_list *error_list;
+struct error_list **error_list_tail = &error_list;
+
+void collect_errors(const char *fmt, va_list params)
+{
+       struct error_list *err = xmalloc(sizeof(*err));
+
+       strbuf_init(&err->buf, 0);
+       strbuf_vaddf(&err->buf, fmt, params);
+       err->next = NULL;
+       *error_list_tail = err;
+       error_list_tail = &err->next;
+}
+
+static void clear_errors(void)
+{
+       while (error_list) {
+               struct error_list *next = error_list->next;
+               strbuf_release(&error_list->buf);
+               free(error_list);
+               error_list = next;
+       }
+       error_list_tail = &error_list;
+}
+
+void print_errors(void (*func)(const char *, ...), const char *fmt, ...)
+{
+       struct strbuf buf = STRBUF_INIT;
+       va_list params;
+       int prefix_len, suffix_start;
+       const char *p;
+       struct error_list *el;
+
+       va_start(params, fmt);
+       strbuf_vaddf(&buf, fmt, params);
+       va_end(params);
+
+       /*
+        * The intent here is that callers will put #{error} in their fmt
+        * string. Doing two layers of interpolation is gross, because we may
+        * accidentally find "#{error}" in one of the substitutions, not the
+        * original fmt. Ideally we would do it all in a single pass (and call
+        * #{error} "%M" or something), but that would require extending
+        * vsprintf, and there is no way to do that portably.
+        */
+       p = strstr(buf.buf, "#{error}");
+       if (!p)
+               die("BUG: error printer does not want to print error!?");
+       prefix_len = p - buf.buf;
+       suffix_start = prefix_len + strlen("#{error}");
+
+       for (el = error_list; el; el = el->next)
+               func("%.*s%.*s%.*s",
+                    prefix_len, buf.buf,
+                    el->buf.len, el->buf.buf,
+                    buf.len - suffix_start, buf.buf + suffix_start);
+
+       strbuf_release(&buf);
+       clear_errors();
 }
 
 void set_die_is_recursing_routine(int (*routine)(void))
@@ -144,7 +227,7 @@ int error(const char *err, ...)
        va_list params;
 
        va_start(params, err);
-       error_routine(err, params);
+       error_funcs->func(err, params);
        va_end(params);
        return -1;
 }
--
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