Fix errno error buffers to not clobber errors

The strerror_r API has two flavors depending on system options.

The bad flavor uses a static buffer for returning results, so if
you save the pointer from strerror_r, the string you're referencing
becomes useless if anybody else calls strerror_r again

The good flavor does what you expect: it writes the error to your buffer.

This commit uses strerror_r directly if it's a good version or copies
the static buffer into our private buffer if it's a bad version.

Thanks to gemorin for explaining the problem and drafting a fix.

Fixes #239

hiredis.c

@@ -579,7 +579,7 @@ void __redisSetError(redisContext *c, int type, const char *str) {
     } else {
         /* Only REDIS_ERR_IO may lack a description! */
         assert(type == REDIS_ERR_IO);
-        strerror_r(errno,c->errstr,sizeof(c->errstr));
+        __redis_strerror_r(errno, c->errstr, sizeof(c->errstr));
     }
 }
 

hiredis.h

@@ -76,6 +76,30 @@
  * SO_REUSEADDR is being used. */
 #define REDIS_CONNECT_RETRIES  10
 
+/* strerror_r has two completely different prototypes and behaviors
+ * depending on system issues, so we need to operate on the error buffer
+ * differently depending on which strerror_r we're using. */
+#ifndef _GNU_SOURCE
+/* "regular" POSIX strerror_r that does the right thing. */
+#define __redis_strerror_r(errno, buf, len)                                    \
+    do {                                                                       \
+        strerror_r((errno), (buf), (len));                                     \
+    } while (0)
+#else
+/* "bad" GNU strerror_r we need to clean up after. */
+#define __redis_strerror_r(errno, buf, len)                                    \
+    do {                                                                       \
+        char *err_str = strerror_r((errno), (buf), (len));                     \
+        /* If return value _isn't_ the start of the buffer we passed in,       \
+         * then GNU strerror_r returned an internal static buffer and we       \
+         * need to copy the result into our private buffer. */                 \
+        if (err_str != (buf)) {                                                \
+            buf[(len)] = '\0';                                                 \
+            strncat((buf), err_str, ((len) - 1));                              \
+        }                                                                      \
+    } while (0)
+#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif

net.c

@@ -67,7 +67,7 @@ static void __redisSetErrorFromErrno(redisContext *c, int type, const char *pref
 
     if (prefix != NULL)
         len = snprintf(buf,sizeof(buf),"%s: ",prefix);
-    strerror_r(errno,buf+len,sizeof(buf)-len);
+    __redis_strerror_r(errno, (char *)(buf + len), sizeof(buf) - len);
     __redisSetError(c,type,buf);
 }