fix(flash): merge segments sharing a flash page

Found on the first real-hardware flash (PM3 Easy). The fullimage ELF has two
adjacent PT_LOAD segments (seg1 ends at 0x15E5AC, seg2 starts there) that share
the 0x15E400 flash page. build_blocks aligned each segment independently, so the
shared page was written twice — seg2's 0xFF-padded head, written last,
erase-programmed the page and wiped seg1's real bytes there (the SAM7 programs a
whole page atomically). That region holds version_information, so the flashed OS
booted but reported 'Missing/Invalid version information'.

Fix: merge segments whose block-aligned spans touch into one buffer, so every
shared page is written once with both segments' real data; 0xFF only fills gaps
and aligned outer edges. Re-flash verified on hardware: 3df2ed2e8 -> 303bd5873,
valid version, matches_pin true. Bootrom untouched throughout (fullimage-only).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pm3py/core/flash.py

@@ -98,19 +98,40 @@ def build_blocks(segments: list[tuple[int, bytes]],
                  block_size: int = FLASH_BLOCK_SIZE) -> list[tuple[int, bytes]]:
     """Turn segments into a list of (address, block) writes.
 
-    Each segment is padded out to ``block_size`` boundaries with 0xFF (erased
-    flash), then split into block-sized chunks. Addresses are block-aligned so
-    each chunk maps onto whole flash pages.
+    Segments that fall in the same or overlapping flash blocks are merged into a
+    single buffer first, so a page shared by two adjacent segments is written
+    once with both segments' real data. Writing such a page twice would be a bug:
+    the SAM7 erase-programs each page atomically, so a second write of the shared
+    page (with 0xFF padding where the other segment's data lives) would wipe the
+    first segment's bytes. Gaps within a merged run and the aligned outer edges
+    are filled with 0xFF (erased flash).
     """
+    if not segments:
+        return []
+    segs = sorted(segments)
+
+    # Group consecutive segments whose block-aligned spans touch or overlap.
+    groups: list[list[tuple[int, bytes]]] = [[segs[0]]]
+    for paddr, data in segs[1:]:
+        g = groups[-1]
+        g_end = max(a + len(d) for a, d in g)
+        if paddr < _align_up(g_end, block_size):
+            g.append((paddr, data))
+        else:
+            groups.append([(paddr, data)])
+
     blocks: list[tuple[int, bytes]] = []
-    for paddr, data in segments:
-        start = _align_down(paddr, block_size)
-        pre = paddr - start
-        end = _align_up(paddr + len(data), block_size)
-        post = end - (paddr + len(data))
-        blob = b"\xFF" * pre + data + b"\xFF" * post
+    for g in groups:
+        g_start = g[0][0]
+        g_end = max(a + len(d) for a, d in g)
+        astart = _align_down(g_start, block_size)
+        aend = _align_up(g_end, block_size)
+        blob = bytearray(b"\xFF" * (aend - astart))
+        for paddr, data in g:
+            off = paddr - astart
+            blob[off:off + len(data)] = data
         for i in range(0, len(blob), block_size):
-            blocks.append((start + i, blob[i:i + block_size]))
+            blocks.append((astart + i, bytes(blob[i:i + block_size])))
     return blocks
 
 

tests/test_flash.py

@@ -105,6 +105,27 @@ def test_build_blocks_aligns_and_pads():
     assert blocks[1][1][-100:] == b"\xFF" * 100
 
 
+def test_build_blocks_merges_shared_page():
+    # Two adjacent segments sharing the flash page at 0x102200: seg1 ends at
+    # 0x1022AC, seg2 starts there. The shared page must keep seg1's real tail —
+    # writing it twice (seg2's 0xFF head last) would wipe it (the on-hardware bug).
+    seg1 = (0x102000, b"\x11" * 0x2AC)   # ends 0x1022AC (unaligned)
+    seg2 = (0x1022AC, b"\x22" * 0x100)   # ends 0x1023AC
+    blocks = dict(build_blocks([seg1, seg2]))
+    shared = blocks[0x102200]            # 0x102200..0x102400
+    assert shared[0x00:0xAC] == b"\x11" * 0xAC   # seg1 tail preserved, NOT 0xFF
+    assert shared[0xAC:0x100] == b"\x22" * 0x54  # seg2 head
+    # no duplicate block addresses (would mean a double-write of a page)
+    addrs = [a for a, _ in build_blocks([seg1, seg2])]
+    assert len(addrs) == len(set(addrs))
+
+
+def test_build_blocks_separate_when_far_apart():
+    # A real gap between segments -> separate groups, no giant fill between them.
+    blocks = build_blocks([(0x102000, b"\x11" * 16), (0x120000, b"\x22" * 16)])
+    assert {a for a, _ in blocks} == {0x102000, 0x120000}
+
+
 def test_build_blocks_pads_low_side():
     # unaligned start -> leading 0xFF padding down to the block boundary
     blocks = build_blocks([(0x102010, b"\x22" * 16)])