docs: add comprehensive debugging analysis for EOF exception fix

Documents the complete debugging journey from initial symptoms through
to the root cause discovery and fix.

Key finding: SeaweedInputStream.read() was returning 0 bytes when copying
inline content, causing Parquet's readFully() to throw EOF exceptions.

The fix ensures read() always returns the actual number of bytes copied.

test/java/spark/DEBUGGING_BREAKTHROUGH.md

@@ -0,0 +1,151 @@
+# Debugging Breakthrough: EOF Exception Analysis
+
+## Summary
+After extensive debugging, we've identified and partially fixed the root cause of the `EOFException: Still have: 78 bytes left` error in Parquet file reads.
+
+## Root Cause Analysis
+
+### Initial Hypothesis ❌ (Incorrect)
+- **Thought**: File size calculation was wrong (`contentLength` off by 78 bytes)
+- **Reality**: `contentLength` was **always correct** at 1275 bytes
+
+### Second Hypothesis ❌ (Partially Correct)
+- **Thought**: `FSDataOutputStream.getPos()` wasn't delegating to `SeaweedOutputStream.getPos()`
+- **Reality**: The override **was working**, but there was a deeper issue
+
+### Third Hypothesis ✅ (ROOT CAUSE)
+- **Problem**: `SeaweedInputStream.read(ByteBuffer buf)` was returning 0 bytes for inline content
+- **Location**: Line 127-129 in `SeaweedInputStream.java`
+- **Bug**: When copying inline content from protobuf entry, `bytesRead` was never updated
+
+```java
+// BEFORE (BUGGY):
+if (this.position < Integer.MAX_VALUE && (this.position + len) <= entry.getContent().size()) {
+    entry.getContent().substring((int) this.position, (int) (this.position + len)).copyTo(buf);
+    // bytesRead stays 0! <-- BUG
+} else {
+    bytesRead = SeaweedRead.read(...);
+}
+return (int) bytesRead; // Returns 0 when inline content was copied!
+```
+
+```java
+// AFTER (FIXED):
+if (this.position < Integer.MAX_VALUE && (this.position + len) <= entry.getContent().size()) {
+    entry.getContent().substring((int) this.position, (int) (this.position + len)).copyTo(buf);
+    bytesRead = len; // FIX: Update bytesRead after inline copy
+} else {
+    bytesRead = SeaweedRead.read(...);
+}
+return (int) bytesRead; // Now returns correct value!
+```
+
+## Why This Caused EOF Errors
+
+1. **Parquet's readFully() loop**:
+   ```java
+   while (remaining > 0) {
+       int read = inputStream.read(buffer, offset, remaining);
+       if (read == -1 || read == 0) {
+           throw new EOFException("Still have: " + remaining + " bytes left");
+       }
+       remaining -= read;
+   }
+   ```
+
+2. **Our bug**: When `read()` returned 0 instead of the actual bytes copied, Parquet thought the stream was done
+3. **Result**: EOF exception with exactly the number of bytes that weren't reported
+
+## Fixes Implemented
+
+### 1. SeaweedInputStream.java (PRIMARY FIX)
+- **File**: `other/java/client/src/main/java/seaweedfs/client/SeaweedInputStream.java`
+- **Change**: Set `bytesRead = len` after inline content copy
+- **Impact**: Ensures `read()` always returns the correct number of bytes read
+
+### 2. SeaweedOutputStream.java (DIAGNOSTIC)
+- **File**: `other/java/client/src/main/java/seaweedfs/client/SeaweedOutputStream.java`
+- **Change**: Added comprehensive logging to `getPos()` with stack traces
+- **Purpose**: Track who calls `getPos()` and what positions are returned
+- **Finding**: All positions appeared correct in tests
+
+### 3. SeaweedFileSystem.java (ALREADY FIXED)
+- **File**: `other/java/hdfs3/src/main/java/seaweed/hdfs/SeaweedFileSystem.java`
+- **Change**: Override `FSDataOutputStream.getPos()` to delegate to `SeaweedOutputStream`
+- **Verification**: Confirmed working with WARN logs
+
+### 4. Unit Test Added
+- **File**: `other/java/client/src/test/java/seaweedfs/client/SeaweedStreamIntegrationTest.java`
+- **Test**: `testRangeReads()` 
+- **Coverage**: 
+  - Range reads at specific offsets (like Parquet footer reads)
+  - Sequential `readFully()` pattern that was failing
+  - Multiple small reads vs. large reads
+  - The exact 78-byte read at offset 1197 that was failing
+
+## Test Results
+
+### Before Fix
+```
+EOFException: Reached the end of stream. Still have: 78 bytes left
+- contentLength: 1275 (correct!)
+- reads: position=1197 len=78 bytesRead=0 ❌
+```
+
+### After Fix  
+```
+No EOF exceptions observed
+- contentLength: 1275 (correct)
+- reads: position=1197 len=78 bytesRead=78 ✅
+```
+
+## Why The 78-Byte Offset Was Consistent
+
+The "78 bytes" wasn't random - it was **systematically the last `read()` call** that returned 0 instead of the actual bytes:
+- File size: 1275 bytes
+- Last read: position=1197, len=78
+- Expected: bytesRead=78
+- Actual (before fix): bytesRead=0
+- Parquet: "I need 78 more bytes but got EOF!" → EOFException
+
+## Commits
+
+1. **e95f7061a**: Fix inline content read bug + add unit test
+2. **c10ae054b**: Add SeaweedInputStream constructor logging  
+3. **5c30bc8e7**: Add detailed getPos() tracking with stack traces
+
+## Next Steps
+
+1. **Push changes** to your branch
+2. **Run CI tests** to verify fix works in GitHub Actions
+3. **Monitor** for any remaining edge cases
+4. **Remove debug logging** once confirmed stable (or reduce to DEBUG level)
+5. **Backport** to other SeaweedFS client versions if needed
+
+## Key Learnings
+
+1. **Read the return value**: Always ensure functions return the correct value, not just perform side effects
+2. **Buffer operations need tracking**: When copying data to buffers, track how much was copied
+3. **Stack traces help**: Knowing WHO calls a function helps understand WHEN bugs occur
+4. **Consistent offsets = systematic bug**: The 78-byte offset being consistent pointed to a logic error, not data corruption
+5. **Downloaded file was perfect**: The fact that `parquet-tools` could read the downloaded file proved the bug was in the read path, not write path
+
+## Files Modified
+
+```
+other/java/client/src/main/java/seaweedfs/client/SeaweedInputStream.java
+other/java/client/src/main/java/seaweedfs/client/SeaweedOutputStream.java  
+other/java/client/src/main/java/seaweedfs/client/SeaweedRead.java
+other/java/client/src/test/java/seaweedfs/client/SeaweedStreamIntegrationTest.java
+other/java/hdfs3/src/main/java/seaweed/hdfs/SeaweedFileSystem.java
+other/java/hdfs3/src/main/java/seaweed/hdfs/SeaweedFileSystemStore.java
+other/java/hdfs3/src/main/java/seaweed/hdfs/SeaweedHadoopOutputStream.java
+```
+
+## References
+
+- Issue: Spark integration tests failing with EOF exception
+- Parquet version: 1.16.0  
+- Spark version: 3.5.0
+- SeaweedFS client version: 3.80.1-SNAPSHOT
+