fix: inconsistent behaviour of BlobInputStream.reset()

A BlobInputStream will start reading from the current position of the LargeObject used to create it, but the current/marked position does not account for this.
Initialising current position correctly will fix this.

Closes pgjdbc#3149

pgjdbc/src/main/java/org/postgresql/largeobject/BlobInputStream.java

@@ -76,7 +76,7 @@ public BlobInputStream(LargeObject lo) {
    */
 
   public BlobInputStream(LargeObject lo, int bsize) {
-    this(lo, bsize, Long.MAX_VALUE);
+    this(lo, bsize, -1);
   }
 
   /**
@@ -90,8 +90,24 @@ public BlobInputStream(LargeObject lo, int bsize, long limit) {
     // The very first read multiplies the last buffer size by two, so we divide by two to get
     // the first read to be exactly the initial buffer size
     this.lastBufferSize = INITIAL_BUFFER_SIZE / 2;
+
+    try {
+      // initialise current position for mark/reset
+      this.absolutePosition = lo.tell64();
+    } catch (SQLException e1) {
+      try {
+        // the tell64 function does not exist before PostgreSQL 9.3
+        this.absolutePosition = lo.tell();
+      } catch (SQLException e2) {
+        RuntimeException e3 = new RuntimeException("Failed to create BlobInputStream", e1);
+        e3.addSuppressed(e2);
+        throw e3;
+      }
+    }
+
     // Treat -1 as no limit for backward compatibility
-    this.limit = limit == -1 ? Long.MAX_VALUE : limit;
+    this.limit = limit == -1 ? Long.MAX_VALUE : limit + this.absolutePosition;
+    this.markPosition = this.absolutePosition;
   }
 
   /**

pgjdbc/src/test/java/org/postgresql/test/jdbc2/BlobTest.java

@@ -207,17 +207,47 @@ void markResetStream() throws Exception {
         LargeObjectManager lom = ((PGConnection) con).getLargeObjectAPI();
 
         long oid = rs.getLong(1);
-        LargeObject blob = lom.open(oid);
-        InputStream bis = blob.getInputStream();
-
-        assertEquals('<', bis.read());
-        bis.mark(4);
-        assertEquals('?', bis.read());
-        assertEquals('x', bis.read());
-        assertEquals('m', bis.read());
-        assertEquals('l', bis.read());
-        bis.reset();
-        assertEquals('?', bis.read());
+        try (LargeObject blob = lom.open(oid)) {
+          InputStream bis = blob.getInputStream();
+
+          assertEquals('<', bis.read());
+          bis.mark(4);
+          assertEquals('?', bis.read());
+          assertEquals('x', bis.read());
+          assertEquals('m', bis.read());
+          assertEquals('l', bis.read());
+          bis.reset();
+          assertEquals('?', bis.read());
+        }
+      }
+    }
+  }
+
+  @Test
+  void markResetWithInitialOffset() throws Exception {
+    assertTrue(uploadFile(TEST_FILE, NATIVE_STREAM) > 0);
+
+    try (Statement stmt = con.createStatement()) {
+      try (ResultSet rs = stmt.executeQuery("SELECT lo FROM testblob where id = '/test-file.xml'")) {
+        assertTrue(rs.next());
+
+        LargeObjectManager lom = ((PGConnection) con).getLargeObjectAPI();
+
+        long oid = rs.getLong(1);
+        try (LargeObject blob = lom.open(oid)) {
+          blob.seek(4);
+          InputStream bis = blob.getInputStream();
+
+          assertEquals('l', bis.read());
+          bis.reset();
+          assertEquals('l', bis.read());
+          assertEquals(' ', bis.read());
+          bis.mark(4);
+          assertEquals('v', bis.read());
+          assertEquals('e', bis.read());
+          bis.reset();
+          assertEquals('v', bis.read());
+        }
       }
     }
   }