chore: temp 26.04 fs probe

Author: not-matthias

.github/workflows/probe2604.yml

@@ -0,0 +1,42 @@
+name: probe2604
+on: [push]
+jobs:
+  probe:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-24.04, ubuntu-26.04]
+    steps:
+      - run: |
+          uname -a
+          for d in / /home/runner/work /home/runner /tmp /var/tmp /mnt; do
+            printf '%s -> %s\n' "$d" "$(stat -f -c %T "$d" 2>/dev/null)"
+          done
+          findmnt -o TARGET,SOURCE,FSTYPE | head -30
+      - name: repro
+        run: |
+          mkdir -p "$HOME/work/probe/target/tmp"
+          cat > /tmp/r.c <<'EOF'
+          #include <stdio.h>
+          #include <stdlib.h>
+          #include <string.h>
+          #include <sys/mman.h>
+          #include <unistd.h>
+          int main(int argc, char** argv) {
+            size_t len = 64UL*1024*1024;
+            char t[4096]; snprintf(t, sizeof t, "%s/probe-XXXXXX", argv[1]);
+            int fd = mkstemp(t); if (fd < 0) return 1; unlink(t);
+            char c[65536]; memset(c, 0x42, sizeof c);
+            for (size_t o = 0; o < len; o += sizeof c) if (write(fd, c, sizeof c) < 0) return 1;
+            void* m = mmap(NULL, len, PROT_READ, MAP_PRIVATE, fd, 0);
+            if (m == MAP_FAILED) return 1;
+            volatile char s = 0;
+            for (size_t i = 0; i < len; i += 4096) s ^= ((volatile char*)m)[i];
+            FILE* f = fopen("/proc/self/status", "r"); char line[256];
+            while (fgets(line, sizeof line, f)) if (strstr(line, "Rss") || strstr(line, "VmHWM")) fputs(line, stdout);
+            return s & 0;
+          }
+          EOF
+          gcc -O0 -o /tmp/r /tmp/r.c
+          for d in "$HOME/work/probe/target/tmp" /var/tmp /tmp; do echo "== $d ($(stat -f -c %T $d))"; /tmp/r "$d"; done