From 50a8a3eb11af29ab97f6d4f9424c19ac2e14d293 Mon Sep 17 00:00:00 2001
From: chrislu <chris.lu@gmail.com>
Date: Mon, 24 Nov 2025 00:35:11 -0800
Subject: [PATCH] docs: comprehensive test results showing unit tests PASS but
 Spark fails
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KEY FINDINGS:
- Unit tests: ALL 3 tests PASS ✅ including exact 78-byte scenario
- getPos() works correctly: returns position + buffer.position()
- FSDataOutputStream override IS being called in Spark
- But EOF exception still occurs at position=1275 trying to read 78 bytes

This proves the bug is NOT in getPos() itself, but in HOW/WHEN Parquet
uses the returned positions.

Hypothesis: Parquet footer has positions recorded BEFORE final flush,
causing a 78-byte offset error in column chunk metadata.
---
 test/java/spark/TEST_RESULTS_SUMMARY.md | 93 +++++++++++++++++++++++++
 1 file changed, 93 insertions(+)
 create mode 100644 test/java/spark/TEST_RESULTS_SUMMARY.md

diff --git a/test/java/spark/TEST_RESULTS_SUMMARY.md b/test/java/spark/TEST_RESULTS_SUMMARY.md
new file mode 100644
index 000000000..a2373b421
--- /dev/null
+++ b/test/java/spark/TEST_RESULTS_SUMMARY.md
@@ -0,0 +1,93 @@
+# Test Results Summary
+
+## Unit Tests: ✅ ALL PASS
+
+Created `GetPosBufferTest` with 3 comprehensive tests that specifically target the Parquet EOF issue:
+
+### Test 1: testGetPosWithBufferedData()
+✅ **PASSED** - Tests basic `getPos()` behavior with multiple writes and buffer management.
+
+### Test 2: testGetPosWithSmallWrites() 
+✅ **PASSED** - Simulates Parquet's pattern of many small writes with frequent `getPos()` calls.
+
+### Test 3: testGetPosWithExactly78BytesBuffered()
+✅ **PASSED** - The critical test that reproduces the EXACT bug scenario!
+
+**Results**:
+```
+Position after 1000 bytes + flush: 1000
+Position with 78 bytes BUFFERED (not flushed): 1078  ✅
+Actual file size: 1078  ✅
+Bytes read at position 1000: 78  ✅
+SUCCESS: getPos() correctly includes buffered data!
+```
+
+## Key Finding
+
+**`getPos()` works correctly in unit tests but Spark tests still fail!**
+
+This proves:
+- ✅ `SeaweedOutputStream.getPos()` returns `position + buffer.position()` correctly
+- ✅ Files are written with correct sizes
+- ✅ Data can be read back at correct positions
+- ✅ The 78-byte buffered scenario works perfectly
+
+## Spark Integration Tests: ❌ STILL FAIL
+
+**BUT** the `FSDataOutputStream.getPos()` override **IS** being called in Spark:
+```
+25/11/24 08:18:56 WARN SeaweedFileSystem: [DEBUG-2024] FSDataOutputStream.getPos() override called! Returning: 0
+25/11/24 08:18:56 WARN SeaweedFileSystem: [DEBUG-2024] FSDataOutputStream.getPos() override called! Returning: 4
+25/11/24 08:18:56 WARN SeaweedFileSystem: [DEBUG-2024] FSDataOutputStream.getPos() override called! Returning: 22
+...
+25/11/24 08:18:56 WARN SeaweedFileSystem: [DEBUG-2024] FSDataOutputStream.getPos() override called! Returning: 190
+```
+
+And the EOF error still occurs:
+```
+position=1275 contentLength=1275 bufRemaining=78
+```
+
+## The Mystery
+
+If `getPos()` is:
+1. ✅ Implemented correctly (unit tests pass)
+2. ✅ Being called by Spark (logs show it)
+3. ✅ Returning correct values (logs show reasonable positions)
+
+**Then why does Parquet still think there are 78 bytes to read at position 1275?**
+
+## Possible Explanations
+
+### Theory 1: Parquet footer writing happens AFTER stream close
+When the stream closes, it flushes the buffer. If Parquet writes the footer metadata BEFORE the final flush but AFTER getting `getPos()`, the footer could have stale positions.
+
+### Theory 2: Buffer position mismatch at close time
+The unit tests show position 1078 with 78 bytes buffered. But when the stream closes and flushes, those 78 bytes get written. If the footer is written based on pre-flush positions, it would be off by 78 bytes.
+
+### Theory 3: Parquet caches getPos() values
+Parquet might call `getPos()` once per column chunk and cache the value. If it caches the value BEFORE the buffer is flushed, but uses it AFTER, the offset would be wrong.
+
+### Theory 4: Multiple streams or file copies
+Spark might be writing to a temporary file, then copying/moving it. If the metadata from the first write is used but the second file is what's read, sizes would mismatch.
+
+## Next Steps
+
+1. **Add logging to close()** - See exact sequence of operations when stream closes
+2. **Add logging to flush()** - See when buffer is actually flushed vs. when getPos() is called
+3. **Check Parquet source** - Understand EXACTLY when it calls getPos() vs. when it writes footer
+4. **Compare with HDFS** - How does HDFS handle this? Does it have special logic?
+
+## Hypothesis
+
+The most likely scenario is that Parquet's `InternalParquetRecordWriter`:
+1. Calls `getPos()` to record column chunk end positions → Gets 1197 (1275 - 78)
+2. Continues writing more data (78 bytes) to buffer
+3. Closes the stream, which flushes buffer (adds 78 bytes)
+4. Final file size: 1275 bytes
+5. But footer says last chunk ends at 1197
+6. So when reading, it tries to read chunk from [1197, 1275) which is correct
+7. BUT it ALSO tries to read [1275, 1353) because it thinks there's MORE data!
+
+**The "78 bytes missing" might actually be "78 bytes DOUBLE-COUNTED"** in the footer metadata!
+