fread() instead of read() + Add GNUC format validator (redis#37)

This commit attempts to reflect also part of the work from previous messy commit:
1. Add GNUC format validator for RDB_log() and RDB_reportError()
2. fread() instead of read() to improve performance.

>> Add GNUC format validator for RDB_log() and RDB_reportError()**
In case of GNUC compiplation add attribute((format(printf, X, Y)))
To verify during compilation correctness of the provided FORMAT
versus variadic arguments that follows.

>> fread() instead of read() to improve performance**
Following a preliminary benchmark that involved reading from a
file-descriptor (fd) I have found out that assisting openfd()
outperform around ~50% better than using lower level read().
This is because when using bare read(), it makes multiple
system-calls to read a small amount of data whereas fread()
attempts to read a big chunk of data to user-space, even if
requested a small amount. I also tested using bare read() with
a buffer that i managed inside readerFileDesc.c and it reached
Similar results to fread().

Note, the benchmark relied on librdb library that is integrated
into another utility (rl_rdbloader), the adjustment is relatively
self-contained and provides a reliable indication of its
effectiveness.

api/librdb-api.h

@@ -11,9 +11,6 @@ extern "C" {
 #define _LIBRDB_API __attribute__((visibility("default")))
 #endif
 
-#ifndef __GNUC__
-#define __attribute__(a)
-#endif
 
 typedef char *RdbBulk;
 typedef char *RdbBulkCopy;
@@ -439,7 +436,12 @@ _LIBRDB_API void RDB_setMaxRawSize(RdbParser *p, size_t maxSize);
 /* logger */
 _LIBRDB_API void RDB_setLogLevel(RdbParser *p, RdbLogLevel l);
 _LIBRDB_API void RDB_setLogger(RdbParser *p, RdbLoggerCB f);
+#ifdef __GNUC__
+_LIBRDB_API void RDB_log(RdbParser *p, RdbLogLevel lvl, const char *format, ...)
+                                          __attribute__((format(printf, 3, 4)));
+#else
 _LIBRDB_API void RDB_log(RdbParser *p, RdbLogLevel lvl, const char *format, ...);
+#endif
 
 /* Following function returns a hint for the total number of items in the current
  * parsed key context - to assist with memory allocation or other optimizations.
@@ -476,8 +478,12 @@ _LIBRDB_API void RDB_pauseParser(RdbParser *p);
  ****************************************************************/
 _LIBRDB_API RdbRes RDB_getErrorCode(RdbParser *p);
 _LIBRDB_API const char *RDB_getErrorMessage(RdbParser *p);
+#ifdef __GNUC__
 _LIBRDB_API void RDB_reportError(RdbParser *p, RdbRes e, const char *msg, ...)
                                         __attribute__((format(printf, 3, 4)));
+#else
+_LIBRDB_API void RDB_reportError(RdbParser *p, RdbRes e, const char *msg, ...);
+#endif
 
 /****************************************************************
  * Memory management

src/ext/readerFileDesc.c

@@ -33,6 +33,7 @@ static RdbStatus readFileDesc(void *data, void *buf, size_t len) {
     size_t totalBytesRead = 0;
 
     while (1) {
+
         totalBytesRead += fread((char *)buf + totalBytesRead, 1, len - totalBytesRead, ctx->file);
 
         if (totalBytesRead == len) {
@@ -75,6 +76,7 @@ RdbxReaderFileDesc *RDBX_createReaderFileDesc(RdbParser *p, int fd, int fdCloseW
         return NULL;
     }
 
+    /* Use fdopen and fread instead of read in order to enjoy read-ahead buffering and less syscalls. */
     FILE *file = fdopen(fd, "r");
 
     if (file == NULL) {

src/lib/parser.c

@@ -469,7 +469,7 @@ _LIBRDB_API void RDB_reportError(RdbParser *p, RdbRes e, const char *msg, ...) {
     vsnprintf(p->errorMsg + nchars, MAX_ERROR_MSG - nchars, msg, args);
     va_end(args);
 
-    RDB_log(p, RDB_LOG_ERR, p->errorMsg);
+    RDB_log(p, RDB_LOG_ERR, "%s", p->errorMsg);
 }
 
 _LIBRDB_API const char *RDB_getErrorMessage(RdbParser *p) {
@@ -659,7 +659,7 @@ static RdbStatus parserMainLoop(RdbParser *p) {
             if (status != RDB_STATUS_OK)
                 at += snprintf(buff+at, sizeof(buff)-1-at, " >>> [Status=%s]", getStatusString(status));
 
-            RDB_log(p, RDB_LOG_DBG, buff);
+            RDB_log(p, RDB_LOG_DBG, "%s", buff);
 
             if (status != RDB_STATUS_OK) break;