zlib: Fix handling of gzip magic bytes mid-file

Only treat the gzip magic bytes, when encountered within the file
after reading a single block, as the start of a new member when
the previous member has ended.

Add test files that reliably reproduce #5852. The gzipped file
in test/fixtures/pseudo-multimember-gzip.gz contains the gzip
magic bytes exactly at the position that node encounters after having
read a single block, leading it to believe that a new data
member is starting.

Fixes: #5852
PR-URL: #5863
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

Author: addaleax

src/node_zlib.cc

@@ -258,7 +258,8 @@ class ZCtx : public AsyncWrap {
           }
         }
         while (ctx->strm_.avail_in >= GZIP_MIN_HEADER_SIZE &&
-               ctx->mode_ == GUNZIP) {
+               ctx->mode_ == GUNZIP &&
+               ctx->err_ == Z_STREAM_END) {
           // Bytes remain in input buffer. Perhaps this is another compressed
           // member in the same archive, or just trailing garbage.
           // Check the header to find out.

test/fixtures/pseudo-multimember-gzip.gz

@@ -4,6 +4,8 @@
 const common = require('../common');
 const assert = require('assert');
 const zlib = require('zlib');
+const path = require('path');
+const fs = require('fs');
 
 const data = Buffer.concat([
   zlib.gzipSync('abc'),
@@ -16,3 +18,23 @@ zlib.gunzip(data, common.mustCall((err, result) => {
   assert.ifError(err);
   assert.equal(result, 'abcdef', 'result should match original string');
 }));
+
+// files that have the "right" magic bytes for starting a new gzip member
+// in the middle of themselves, even if they are part of a single
+// regularly compressed member
+const pmmFileZlib = path.join(common.fixturesDir, 'pseudo-multimember-gzip.z');
+const pmmFileGz = path.join(common.fixturesDir, 'pseudo-multimember-gzip.gz');
+
+const pmmExpected = zlib.inflateSync(fs.readFileSync(pmmFileZlib));
+const pmmResultBuffers = [];
+
+fs.createReadStream(pmmFileGz)
+  .pipe(zlib.createGunzip())
+  .on('error', (err) => {
+    assert.ifError(err);
+  })
+  .on('data', (data) => pmmResultBuffers.push(data))
+  .on('finish', common.mustCall(() => {
+    assert.deepStrictEqual(Buffer.concat(pmmResultBuffers), pmmExpected,
+      'result should match original random garbage');
+  }));