Trying out <stdatomic.h> for port/atomics.h (opt-in alternative) [cf-7005]

.clangd

@@ -0,0 +1,43 @@
+Diagnostics:
+  MissingIncludes: None
+InlayHints:
+  Enabled: true
+  ParameterNames: true
+  DeducedTypes: true
+CompileFlags:
+  CompilationDatabase: build/       # Search build/ directory for compile_commands.json
+  Remove: [ -Werror ]
+  Add:
+  - -DDEBUG
+  - -DLOCAL
+  - -DPGDLLIMPORT=
+  - -DPIC
+  - -O2
+  - -Wall
+  - -Wcast-function-type
+  - -Wconversion
+  - -Wdeclaration-after-statement
+  - -Wendif-labels
+  - -Werror=vla
+  - -Wextra
+  - -Wfloat-equal
+  - -Wformat-security
+  - -Wimplicit-fallthrough=3
+  - -Wmissing-format-attribute
+  - -Wmissing-prototypes
+  - -Wno-format-truncation
+  - -Wno-sign-conversion
+  - -Wno-stringop-truncation
+  - -Wno-unused-const-variable
+  - -Wpointer-arith
+  - -Wshadow
+  - -Wshadow=compatible-local
+  - -fPIC
+  - -fexcess-precision=standard
+  - -fno-strict-aliasing
+  - -fvisibility=hidden
+  - -fwrapv
+  - -g
+  - -std=c11
+  - -I.
+  - -I../../../../src/include

.envrc

@@ -0,0 +1,10 @@
+# Inherit shared CFLAGS from ~/ws/postgres/.envrc, load this worktree's
+# flake, then layer the postgres agent context (helpers need flake's PATH +
+# write into this worktree's dir).
+source_up
+watch_file flake.nix
+use flake
+use project_steering c aws
+use project_steering postgresql
+use project_mcp postgresq
+use project_mcp context7

.gdbinit

@@ -0,0 +1,156 @@
+# HOT Indexed Updates — GDB breakpoints for code review
+#
+# Usage:  gdb -x .gdbinit <postgres-binary>
+# Or from gdb:  source .gdbinit
+#
+# These breakpoints cover the major code paths introduced or modified by
+# the HOT indexed updates patch series.  They are organized by subsystem
+# to make it easy to enable/disable groups during debugging.
+#
+# Tip: to skip to a specific subsystem, disable all then enable selectively:
+#   disable breakpoints
+#   enable 1 2 3          # just the update-decision group
+
+# =========================================================================
+# 1. UPDATE DECISION — heap_update() HOT/HOT-indexed/non-HOT choice
+#    src/backend/access/heap/heapam.c
+# =========================================================================
+
+# Main entry: heap_update
+break heapam.c:3210
+
+# HOT decision block: pure HOT vs HOT indexed vs non-HOT
+# Line 4019: pure HOT (no indexed columns changed)
+# Line 4024: HOT indexed path (non-catalog, some indexed columns changed)
+# Line 4031: predict augmented tuple size
+# Line 4033: size+space check before creating augmented tuple
+break heapam.c:4019
+break heapam.c:4024
+break heapam.c:4033
+
+# Set HEAP_INDEXED_UPDATED flag on new tuple before page insertion
+break heapam.c:4101
+
+# Restore HEAP_INDEXED_UPDATED on old tuple (only if it previously had it)
+break heapam.c:4147
+
+# =========================================================================
+# 2. TUPLE CREATION — building the augmented tuple with embedded bitmap
+#    src/backend/access/heap/heapam.c
+# =========================================================================
+
+# Predict augmented tuple size (returns 0 if t_hoff would overflow)
+break heap_hot_indexed_tuple_size
+
+# Create augmented tuple with embedded modified-column bitmap
+break heap_hot_indexed_create_tuple
+
+# Serialize Bitmapset into raw bytes in tuple header
+break heap_hot_indexed_serialize_bitmap
+
+# =========================================================================
+# 3. BITMAP UTILITIES — raw bitmap operations for chain following
+#    src/backend/access/heap/heapam.c
+# =========================================================================
+
+# Compute raw bitmap byte size from natts
+break heap_hot_indexed_bitmap_raw_size
+
+# Check if tuple header has room for bitmap between null bitmap and data
+break heap_hot_indexed_has_bitmap_space
+
+# Read HOT indexed bitmap from tuple header (returns Bitmapset)
+break heap_hot_indexed_read_bitmap
+
+# Fast overlap check: does tuple's raw bitmap overlap with indexed_attrs?
+break heap_hot_indexed_bitmap_overlaps_raw
+
+# OR a tuple's raw bitmap into an accumulator buffer
+break heap_hot_indexed_bitmap_or_raw
+
+# Check if accumulated raw bitmap overlaps with indexed_attrs
+break heap_hot_indexed_accum_overlaps
+
+# Merge bitmaps from dead tuples into a target tuple on the page
+break heap_hot_indexed_merge_bitmaps_raw
+
+# Deserialize raw bytes back to Bitmapset
+break heap_hot_indexed_deserialize_bitmap
+
+# =========================================================================
+# 4. INDEX SCAN — HOT chain following with stale-entry detection
+#    src/backend/access/heap/heapam_indexscan.c
+# =========================================================================
+
+# Main HOT chain search with indexed update awareness
+break heap_hot_search_buffer
+
+# Redirect-with-data: initialize bitmap accumulator from collapsed redirect
+break heapam_indexscan.c:182
+
+# Accumulate bitmap from INDEXED_UPDATED tuple in chain
+break heapam_indexscan.c:250
+
+# Stale entry detection: accumulated bitmap overlaps this index's attrs
+break heapam_indexscan.c:297
+
+# =========================================================================
+# 5. INDEX SCAN SETUP — indexed_attrs bitmap computation
+#    src/backend/access/index/indexam.c
+# =========================================================================
+
+# Compute indexed_attrs for HOT indexed update chain following
+break indexam.c:299
+
+# =========================================================================
+# 6. INDEX INSERTION — skip unchanged indexes for HOT indexed updates
+#    src/backend/executor/execIndexing.c
+# =========================================================================
+
+# Entry: insert/update index tuples
+break ExecInsertIndexTuples
+
+# Index skip decision: skip indexes whose attrs don't overlap modified set
+break execIndexing.c:370
+
+# =========================================================================
+# 7. PRUNING — chain collapsing and redirect-with-data
+#    src/backend/access/heap/pruneheap.c
+# =========================================================================
+
+# Main prune function
+break heap_page_prune_and_freeze
+
+# Per-chain pruning entry
+break heap_prune_chain
+
+# Chain collapsing: collect bitmaps from dead INDEXED_UPDATED intermediates
+break pruneheap.c:1802
+
+# OR dead tuple bitmaps into combined bitmap
+break pruneheap.c:1836
+
+# Record redirect-with-data for execute phase
+break pruneheap.c:1863
+
+# Execute phase: apply redirect-with-data entries on the page
+break pruneheap.c:1287
+
+# =========================================================================
+# 8. WAL REPLAY — recovery of HOT indexed updates
+#    src/backend/access/heap/heapam_xlog.c
+# =========================================================================
+
+# WAL replay for XLOG_HEAP2_INDEXED_UPDATE
+break heap_xlog_indexed_update
+
+# =========================================================================
+# 9. WAL LOGGING — writing HOT indexed update records
+#    src/backend/access/heap/heapam.c
+# =========================================================================
+
+# WAL logging for heap updates (handles indexed_update flag)
+break log_heap_update
+
+# Serialize redirect-with-data into WAL record (pruneheap.c)
+break pruneheap.c:2936

.local-gitignore

@@ -0,0 +1,3 @@
+# git config core.excludesFile .local-gitignore
+.local-gitignore
+

.mcp.json

@@ -0,0 +1,15 @@
+{
+  "mcpServers": {
+    "postgresq": {
+      "type": "http",
+      "url": "https://pg.ddx.io/mcp/"
+    },
+    "context7": {
+      "args": [
+        "-y",
+        "@upstash/context7-mcp@latest"
+      ],
+      "command": "npx"
+    }
+  }
+}

AGENTS.md

@@ -0,0 +1,3 @@
+# Project instructions
+
+See .agent-steering-domains.md for domain-specific steering (local).

configure

@@ -4918,6 +4918,13 @@ fi
 
 
 
+
+#
+# C11 stdatomic.h
+#
+# stdatomic.h is the sole atomics implementation.  The actual detection runs
+# later, after the compiler and 64-bit integer handling have been configured;
+# a missing or broken header is a hard error there.
 for ac_prog in gawk mawk nawk awk
 do
   # Extract the first word of "$ac_prog", so it can be a program name with args.
@@ -17701,6 +17708,84 @@ $as_echo "#define HAVE_GCC__ATOMIC_INT64_CAS 1" >>confdefs.h
 fi
 
 
+# Require a working C11 stdatomic.h: it is the sole atomics implementation,
+# so a missing or broken header is a hard error.  Use a link test, not a
+# compile test: 64-bit atomic_compare_exchange_strong() can lower to a
+# libatomic call (__atomic_compare_exchange_8) on some 32-bit or
+# weakly-aligned targets, where a compile-only check would pass and then fail
+# at final link.  Retry with -latomic if the bare link fails.
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for working C11 stdatomic.h" >&5
+$as_echo_n "checking for working C11 stdatomic.h... " >&6; }
+if ${pgac_cv_stdatomic+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  pgac_cv_stdatomic=no
+ cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+#include <stdatomic.h>
+int
+main ()
+{
+_Atomic int x = 0;
+    atomic_fetch_add(&x, 1);
+    atomic_thread_fence(memory_order_seq_cst);
+    _Atomic(unsigned long long) y = 0;
+    unsigned long long expected = 0;
+    atomic_compare_exchange_strong(&y, &expected, 42);
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_link "$LINENO"; then :
+  pgac_cv_stdatomic=yes
+else
+  pgac_save_LIBS=$LIBS
+   LIBS="$LIBS -latomic"
+   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+#include <stdatomic.h>
+int
+main ()
+{
+_Atomic int x = 0;
+      atomic_fetch_add(&x, 1);
+      atomic_thread_fence(memory_order_seq_cst);
+      _Atomic(unsigned long long) y = 0;
+      unsigned long long expected = 0;
+      atomic_compare_exchange_strong(&y, &expected, 42);
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_link "$LINENO"; then :
+  pgac_cv_stdatomic='yes, with -latomic'
+else
+  pgac_cv_stdatomic=no
+fi
+rm -f core conftest.err conftest.$ac_objext \
+    conftest$ac_exeext conftest.$ac_ext
+   LIBS=$pgac_save_LIBS
+fi
+rm -f core conftest.err conftest.$ac_objext \
+    conftest$ac_exeext conftest.$ac_ext
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $pgac_cv_stdatomic" >&5
+$as_echo "$pgac_cv_stdatomic" >&6; }
+
+if test "$pgac_cv_stdatomic" = no; then
+  as_fn_error $? "a working C11 stdatomic.h is required (it is the only atomics implementation)" "$LINENO" 5
+fi
+
+
+$as_echo "#define USE_STDATOMIC_H 1" >>confdefs.h
+
+if test "$pgac_cv_stdatomic" = 'yes, with -latomic'; then
+  LIBS="$LIBS -latomic"
+fi
+
+
 # Check for __get_cpuid() and __cpuid()
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for __get_cpuid" >&5
 $as_echo_n "checking for __get_cpuid... " >&6; }

configure.ac

@@ -443,6 +443,13 @@ choke me
 PGAC_ARG_BOOL(with, llvm, no, [build with LLVM based JIT support],
               [AC_DEFINE([USE_LLVM], 1, [Define to 1 to build with LLVM based JIT support. (--with-llvm)])])
 AC_SUBST(with_llvm)
+
+#
+# C11 stdatomic.h
+#
+# stdatomic.h is the sole atomics implementation.  The actual detection runs
+# later, after the compiler and 64-bit integer handling have been configured;
+# a missing or broken header is a hard error there.
 dnl must use AS_IF here, else AC_REQUIRES inside PGAC_LLVM_SUPPORT malfunctions
 AS_IF([test "$with_llvm" = yes], [
   PGAC_LLVM_SUPPORT()
@@ -2112,6 +2119,48 @@ PGAC_HAVE_GCC__ATOMIC_INT32_CAS
 PGAC_HAVE_GCC__ATOMIC_INT64_CAS
 
 
+# Require a working C11 stdatomic.h: it is the sole atomics implementation,
+# so a missing or broken header is a hard error.  Use a link test, not a
+# compile test: 64-bit atomic_compare_exchange_strong() can lower to a
+# libatomic call (__atomic_compare_exchange_8) on some 32-bit or
+# weakly-aligned targets, where a compile-only check would pass and then fail
+# at final link.  Retry with -latomic if the bare link fails.
+AC_CACHE_CHECK([for working C11 stdatomic.h], [pgac_cv_stdatomic],
+[pgac_cv_stdatomic=no
+ AC_LINK_IFELSE([AC_LANG_PROGRAM([#include <stdatomic.h>],
+  [[_Atomic int x = 0;
+    atomic_fetch_add(&x, 1);
+    atomic_thread_fence(memory_order_seq_cst);
+    _Atomic(unsigned long long) y = 0;
+    unsigned long long expected = 0;
+    atomic_compare_exchange_strong(&y, &expected, 42);
+  ]])],
+  [pgac_cv_stdatomic=yes],
+  [pgac_save_LIBS=$LIBS
+   LIBS="$LIBS -latomic"
+   AC_LINK_IFELSE([AC_LANG_PROGRAM([#include <stdatomic.h>],
+    [[_Atomic int x = 0;
+      atomic_fetch_add(&x, 1);
+      atomic_thread_fence(memory_order_seq_cst);
+      _Atomic(unsigned long long) y = 0;
+      unsigned long long expected = 0;
+      atomic_compare_exchange_strong(&y, &expected, 42);
+    ]])],
+    [pgac_cv_stdatomic='yes, with -latomic'],
+    [pgac_cv_stdatomic=no])
+   LIBS=$pgac_save_LIBS])])
+
+if test "$pgac_cv_stdatomic" = no; then
+  AC_MSG_ERROR([a working C11 stdatomic.h is required (it is the only atomics implementation)])
+fi
+
+AC_DEFINE([USE_STDATOMIC_H], 1,
+          [Define to 1 to use C11 stdatomic.h for atomic operations.])
+if test "$pgac_cv_stdatomic" = 'yes, with -latomic'; then
+  LIBS="$LIBS -latomic"
+fi
+
+
 # Check for __get_cpuid() and __cpuid()
 AC_CACHE_CHECK([for __get_cpuid], [pgac_cv__get_cpuid],
 [AC_LINK_IFELSE([AC_LANG_PROGRAM([#include <cpuid.h>],