[PATCH 2/3] Better error reporting afer adding advanced error messages

[Bodo Eggert]

The previous patch was a simpla patch to add advanced error messafges. It
was intentionally dumb at outputting the message to keep the changes to a
minimum.

This patch adds a more smart function to either use perror or to print the
advanced error message or sould both be of relevance - print both.

EINVAL is expected to be useless to the reader, and errno == 0 is expected
to be from an unsuccessful call to some internal function that was not
caused by an error while calling a system library function.

The advanced error 1 is still reserved for generic failures, for now it is
used for generic parse errors. I did not yet concern myself with these.
Since that code isn't meaningful to the user, it's not printed.
---
 tools/parse.c   | 12 +--------
 tools/setfacl.c | 66 +++++++++++++++++++++++++++++++++++++++++++------
 2 files changed, 59 insertions(+), 19 deletions(-)

diff --git a/tools/parse.c b/tools/parse.c
index b4d0b7d..6f12ec6 100644
--- a/tools/parse.c
+++ b/tools/parse.c
@@ -42,7 +42,7 @@
                (x)++; \
        })
 
-/*char *advanced_errors[] = {
+char *advanced_errors[] = {
        "OK",
        "(generic)",
        "default acl exists",
@@ -50,18 +50,8 @@
        "Error while looking up user",
        "Group not found",
        "Error while looking up group",
-};*/
-char *advanced_errors[] = {
-       "",
-       "",
-       " (default acl exists)",
-       " (User not found)",
-       " (Error while looking up user)",
-       " (Group not found)",
-       " (Error while looking up group)",
 };
 
-
 static int
 skip_tag_name(
        const char **text_p,
diff --git a/tools/setfacl.c b/tools/setfacl.c
index fe2f79d..df9695a 100644
--- a/tools/setfacl.c
+++ b/tools/setfacl.c
@@ -111,7 +111,57 @@ has_any_of_type(
        }
        return 0;
 }
-       
+
+
+// There might be a library function that's more efficient
+// for now we are required to use the reference implementation
+char *
+stpecpy(char *dst, char end[0], const char *restrict src)
+__attribute__((weak));
+
+/* This code is in the public domain,
+   it is copied from Linux man-pages 6.04 */
+#pragma GCC diagnostic push
+#pragma GCC diagnostic error "-Wstringop-overflow=0"
+/* The end pointer is expected to be the end of the buffer
+   so it's safe to access despite being char[0] */
+char *
+stpecpy(char *dst, char end[0], const char *restrict src)
+{
+       char *p;
+
+       if (dst == NULL)
+               return NULL;
+       if (dst == end)
+               return end;
+
+       p = memccpy(dst, src, '\0', end - dst);
+       if (p != NULL)
+               return p - 1;
+
+       /* truncation detected */
+       end[-1] = '\0';
+       return end;
+}
+#pragma GCC diagnostic pop
+
+char const *
+get_advanced_error(
+       int xerrno, // errno is a macro
+       int err_ret)
+{
+       static char buf[4096];
+       char * end = buf + sizeof(buf);
+       if (err_ret <= 1)
+               return strerror(xerrno);
+       if (!xerrno || (xerrno == EINVAL))
+               return advanced_errors[err_ret];
+       char * tmp = stpecpy(buf, end, strerror(xerrno));
+       tmp = stpecpy(tmp, end, " (");
+       tmp = stpecpy(tmp, end, advanced_errors[err_ret]);
+       tmp = stpecpy(tmp, end, ")");
+       return buf;
+}
 
 #if !POSIXLY_CORRECT
 int
@@ -171,9 +221,9 @@ restore(
                                     SEQ_PARSE_MULTI,
                                     &lineno, NULL, &err_ret);
                if (error != 0) {
-                       fprintf(stderr, _("%s: %s: %s%s in line %d\n"),
-                               progname, xquote(filename, "\n\r"), 
strerror(errno),
-                               advanced_errors[err_ret], lineno);
+                       fprintf(stderr, _("%s: %s: %s in line %d\n"),
+                               progname, xquote(filename, "\n\r"),
+                               get_advanced_error(errno, err_ret), lineno);
                        status = 1;
                        goto getout;
                }
@@ -456,10 +506,10 @@ int main(int argc, char *argv[])
                                        } else {
                                                fprintf(stderr, _(
                                                        "%s: Option "
-                                                       "-%c: %s%s near "
+                                                       "-%c: %s near "
                                                        "character %d\n"),
                                                        progname, opt,
-                                                       strerror(errno), 
advanced_errors[err_ret],
+                                                       
get_advanced_error(errno, err_ret),
                                                        which+1);
                                        }
                                        status = 2;
@@ -534,7 +584,7 @@ int main(int argc, char *argv[])
                                                        "%s: %s%s in line "
                                                        "%d of file %s\n"),
                                                        progname,
-                                                       strerror(errno), 
advanced_errors[err_ret],
+                                                       
get_advanced_error(errno, err_ret),
                                                        lineno,
                                                        xquote(optarg, "\n\r"));
                                        } else {
@@ -542,7 +592,7 @@ int main(int argc, char *argv[])
                                                        "%s: %s%s in line "
                                                        "%d of standard "
                                                        "input\n"), progname,
-                                                       strerror(errno), 
advanced_errors[err_ret],
+                                                       
get_advanced_error(errno, err_ret),
                                                        lineno);
                                        }
                                        status = 2;
-- 
2.42.0