fwrite(): when col.names=FALSE, produce a gzip header, avoid a leak

NEWS.md

@@ -10,6 +10,8 @@
 
 1. Custom binary operators from the `lubridate` package now work with objects of class `IDate` as with a `Date` subclass, [#6839](https://github.com/Rdatatable/data.table/issues/6839). Thanks @emallickhossain for the report and @aitap for the fix.
 
+2. `fwrite(compress="gzip")` once again produces a gzip header when the column names are missing or disabled, [@6852](https://github.com/Rdatatable/data.table/issues/6852). Thanks @maxscheiber for the report and @aitap for the fix.
+
 ## NOTES
 
 1. Continued work to remove non-API C functions, [#6180](https://github.com/Rdatatable/data.table/issues/6180). Thanks Ivan Krylov for the PRs and for writing a clear and concise guide about the R API: https://aitap.codeberg.page/R-api/.

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);
@@ -898,21 +912,26 @@ 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
       }
     }
   }
+#ifndef NOZLIB
+  else {
+    // was unconditionally initialized for zbuffSize, not used for header
+    deflateEnd(&strm);
+  }
+#endif
   if (verbose)
     DTPRINT(_("Initialization done in %.3fs\n"), 1.0*(wallclock()-t0));