refactor: apply JPEGDEC patches via git apply, not string replace

The previous patch_jpegdec.py used in-place string replacement with a single shared marker for two distinct DC writes (main store and successive-approximation update). If only one of the two anchors matched, the file was written half-patched and the shared marker locked the partial state in for every subsequent run. Generate the fixes as `git format-patch` artifacts under scripts/jpegdec_patches/, then apply them in lexical order via `git apply`. Idempotency is decided by git itself: `--check --reverse` succeeds means already applied; `--check` succeeds means appliable; neither aborts the build rather than leaving a half-patched file. No behaviour change to the patched JPEGDEC source: same redirect, same DC guards, same intent. Just stops the pre-build script from doing something it has no business doing.
2026-05-19 20:05:07 +02:00
parent 597edeebd8
commit 9e68ced046
3 changed files with 128 additions and 99 deletions
@@ -0,0 +1,28 @@
+From 5dff5afab0c68d0d0c4385d72e6f0c030b613960 Mon Sep 17 00:00:00 2001
+From: patch <patch@local>
+Date: Mon, 18 May 2026 20:57:54 -0700
+Subject: [PATCH 1/2] Redirect pMCU to sMCUs[0] when iMCU < 0 (MCU_SKIP)
+
+---
+ src/jpeg.inl | 5 ++++-
+ 1 file changed, 4 insertions(+), 1 deletion(-)
+
+diff --git a/src/jpeg.inl b/src/jpeg.inl
+index a60b548..26bcf6f 100644
+--- a/src/jpeg.inl
+++ b/src/jpeg.inl
+@@ -1824,7 +1824,10 @@ static int JPEGDecodeMCU_P(JPEGIMAGE *pJPEG, int iMCU, int *iDCPredictor)
+     unsigned short *pFast;
+     uint32_t usHuff; // this prevents an unnecessary & 65535 for shorts
+     signed int iPositive, iNegative, iCoeff;
+-    signed short *pMCU = &pJPEG->sMCUs[iMCU & 0xffffff];
+    // CrossPoint patch: redirect pMCU to sMCUs[0] when MCU_SKIP to avoid
+    // a wild pointer (~33 MB past sMCUs) that store-faults on AC writes.
+    signed short *pMCU = (iMCU < 0) ? pJPEG->sMCUs
+                                    : &pJPEG->sMCUs[iMCU & 0xffffff];
+     uint32_t ulBitOff;
+     my_ulong ulCode, ulBits, ulTemp; // local copies to allow compiler to use register vars
+     uint8_t *pBuf;
+-- 
+2.50.1 (Apple Git-155)
+
@@ -0,0 +1,41 @@
+From f6238b54c2de34c8e29b0d371a7e902d9cf579bf Mon Sep 17 00:00:00 2001
+From: patch <patch@local>
+Date: Mon, 18 May 2026 20:58:22 -0700
+Subject: [PATCH 2/2] Guard pMCU[0] DC writes against MCU_SKIP
+
+---
+ src/jpeg.inl | 11 +++++++++--
+ 1 file changed, 9 insertions(+), 2 deletions(-)
+
+diff --git a/src/jpeg.inl b/src/jpeg.inl
+index 26bcf6f..1bd38b2 100644
+--- a/src/jpeg.inl
+++ b/src/jpeg.inl
+@@ -1855,7 +1855,11 @@ static int JPEGDecodeMCU_P(JPEGIMAGE *pJPEG, int iMCU, int *iDCPredictor)
+             {
+                 //            (*iDCPredictor) |= iPositive;  // in case the scan is run more than once
+                 //            pMCU[0] = *iDCPredictor; // store in MCU[0]
+-                pMCU[0] |= iPositive;
+                // CrossPoint patch: guard against MCU_SKIP. The pMCU
+                // redirect makes &sMCUs[0] safe to dereference, but
+                // writing here would clobber the just-decoded Y DC.
+                if (iMCU >= 0)
+                    pMCU[0] |= iPositive;
+             }
+             goto mcu_done; // that's it
+         }
+@@ -1887,7 +1891,10 @@ static int JPEGDecodeMCU_P(JPEGIMAGE *pJPEG, int iMCU, int *iDCPredictor)
+             ulCode <<= pJPEG->cApproxBitsLow; // successive approximation shift value
+             (*iDCPredictor) += ulCode;
+         }
+-        pMCU[0] = (short)*iDCPredictor; // store in MCU[0]
+        // CrossPoint patch: guard against MCU_SKIP. See note on the
+        // matching SA write above.
+        if (iMCU >= 0)
+            pMCU[0] = (short)*iDCPredictor; // store in MCU[0]
+     }
+     // Now get the other 63 AC coefficients
+     pFast = &pJPEG->usHuffAC[pJPEG->ucACTable * HUFF11SIZE];
+-- 
+2.50.1 (Apple Git-155)
+