fwrite(): emit gzip header even if !col.names

Previously, fwrite() emitted a raw DEFLATE stream if col.names was FALSE
(and thus headerLen was zero). Decompressing such files is possible, but
not trivial.

Author: aitap

inst/tests/tests.Rraw

@@ -10024,9 +10024,12 @@ if (!haszlib()) {
   fwrite(DT, file=f3<-tempfile(), compress="gzip")   # compress to filename not ending .gz
   fwrite(DT, file=f4<-tempfile(), compress="gzip", compressLevel=1)   # test compressLevel
   fwrite(DT, file=f5<-tempfile(), compress="gzip", compressLevel=9)
+  # col.names=FALSE must not disable gzip header, #6852
+  fwrite(DT, file=f6<-tempfile(), compress="gzip", col.names=FALSE)
   test(1658.441, file.info(f3)$size, file.info(f1)$size)
   test(1658.442, file.info(f4)$size >= file.info(f1)$size)
   test(1658.443, file.info(f1)$size >= file.info(f5)$size)
+  test(1658.444, fread(f6, col.names = c("a", "b")), DT)
   unlink(c(f1,f2,f3,f4,f5))
 }
 DT = data.table(a=1:3, b=list(1:4, c(3.14, 100e10), c("foo", "bar", "baz")))

src/fwrite.c

@@ -829,8 +829,27 @@ void fwriteMain(fwriteMainArgs args)
            zbuffSize / MEGA, nth, errno, strerror(errno));
       // # nocov end
     }
-  }
+    len = 0;
+    crc = crc32(0L, Z_NULL, 0);
+
+    if (f != -1) {
+      // write minimal gzip header, but not on the console
+      static const char header[] = "\037\213\10\0\0\0\0\0\0\3";
+      int ret0 = WRITE(f, header, (sizeof header) - 1);
+      compress_len += (sizeof header) - 1;
 
+      if (ret0 == -1) {
+        // # nocov start
+        int errwrite = errno; // capture write errno now in case close fails with a different errno
+        CLOSE(f);
+        free(buffPool);
+        free(zbuffPool);
+        deflateEnd(&strm);
+        STOP(_("Failed to write gzip header. Write returned %d"), ret0);
+        // # nocov end
+      }
+    }
+  }
 #endif
 
   // write header
@@ -873,15 +892,10 @@ void fwriteMain(fwriteMainArgs args)
       *ch = '\0';
       DTPRINT("%s", buff); // # notranslate
     } else {
-      int ret0=0, ret1=0, ret2=0;
+      int ret1=0, ret2=0;
 #ifndef NOZLIB
       if (args.is_gzip) {
         char* zbuff = zbuffPool;
-        // write minimal gzip header
-        char* header = "\037\213\10\0\0\0\0\0\0\3";
-        ret0 = WRITE(f, header, 10);
-        compress_len += 10;
-        crc = crc32(0L, Z_NULL, 0);
 
         size_t zbuffUsed = zbuffSize;
         len = (size_t)(ch - buff);
@@ -898,16 +912,15 @@ void fwriteMain(fwriteMainArgs args)
 #ifndef NOZLIB
       }
 #endif
-      if (ret0 == -1 || ret1 || ret2 == -1) {
+      if (ret1 || ret2 == -1) {
         // # nocov start
         int errwrite = errno; // capture write errno now in case close fails with a different errno
         CLOSE(f);
         free(buffPool);
 #ifndef NOZLIB
         free(zbuffPool);
 #endif
-        if (ret0 == -1) STOP(_("Failed to write gzip header. Write returned %d"), ret0);
-        else if (ret1) STOP(_("Failed to compress gzip. compressbuff() returned %d"), ret1);
+        if (ret1) STOP(_("Failed to compress gzip. compressbuff() returned %d"), ret1);
         else STOP(_("%s: '%s'"), strerror(errwrite), args.filename);
         // # nocov end
       }