Fix page cache reclaim test when /tmp is tmpfs (#16067)

### Issue

`test_reclaimFilePageCache` fails when `/tmp` is mounted as tmpfs, as on
Ubuntu 26.04. `POSIX_FADV_DONTNEED` does not evict tmpfs file pages, so
the cache reclaim assertions fail. The daily `test-ubuntu-reclaim-cache`
job has the same problem with RDB files under `/tmp`.

### Change

Use `/var/tmp`, which normally uses a disk filesystem, for the unit test
file and the daily job's master and replica directories. Update the
related pidfile and RDB paths. Keep the existing cache reclaim checks.

.github/workflows/daily.yml

@@ -500,8 +500,8 @@ jobs:
     - name: testprep
       run: |
         sudo apt-get install vmtouch
-        mkdir /tmp/master 
-        mkdir /tmp/slave
+        mkdir /var/tmp/master
+        mkdir /var/tmp/slave
     - name: warm up
       run: |
         ./src/redis-server --daemonize yes --logfile /dev/null
@@ -535,16 +535,16 @@ jobs:
         echo "test SAVE doesn't increase cache"
         CACHE0=$(grep -w file /sys/fs/cgroup/memory.stat | awk '{print $2}')
         echo "$CACHE0"
-        ./src/redis-server --daemonize yes --logfile /dev/null --dir /tmp/master --port 8080 --repl-diskless-sync no --pidfile /tmp/master/redis.pid --rdbcompression no --enable-debug-command yes
+        ./src/redis-server --daemonize yes --logfile /dev/null --dir /var/tmp/master --port 8080 --repl-diskless-sync no --pidfile /var/tmp/master/redis.pid --rdbcompression no --enable-debug-command yes
         
         # Runtime TSC calibration can delay startup on x86, while the daemonized
         # parent exits before the child listens. Wait before the first command.
         wait_for_redis_start 8080
         ./src/redis-cli -p 8080 debug populate 10000 k 102400
-        ./src/redis-server --daemonize yes --logfile /dev/null --dir /tmp/slave --port 8081 --repl-diskless-load disabled --rdbcompression no
+        ./src/redis-server --daemonize yes --logfile /dev/null --dir /var/tmp/slave --port 8081 --repl-diskless-load disabled --rdbcompression no
         wait_for_redis_start 8081
         ./src/redis-cli -p 8080 save > /dev/null
-        VMOUT=$(vmtouch -v /tmp/master/dump.rdb)
+        VMOUT=$(vmtouch -v /var/tmp/master/dump.rdb)
         echo $VMOUT
         grep -q " 0%" <<< $VMOUT 
         CACHE=$(grep -w file /sys/fs/cgroup/memory.stat | awk '{print $2}')
@@ -555,10 +555,10 @@ jobs:
         ./src/redis-cli -p 8081 REPLICAOF 127.0.0.1 8080 > /dev/null
         while [ $(./src/redis-cli -p 8081 info replication | grep "master_link_status:down") ]; do sleep 1; done;
         sleep 1 # wait for the completion of cache reclaim bio
-        VMOUT=$(vmtouch -v /tmp/master/dump.rdb)
+        VMOUT=$(vmtouch -v /var/tmp/master/dump.rdb)
         echo $VMOUT
         grep -q " 0%" <<< $VMOUT 
-        VMOUT=$(vmtouch -v /tmp/slave/dump.rdb)
+        VMOUT=$(vmtouch -v /var/tmp/slave/dump.rdb)
         echo $VMOUT
         grep -q " 0%" <<< $VMOUT 
         CACHE=$(grep -w file /sys/fs/cgroup/memory.stat | awk '{print $2}')
@@ -566,10 +566,10 @@ jobs:
         if [ "$(( $CACHE-$CACHE0 ))" -gt "8000000" ]; then exit 1; fi
         
         echo "test reboot doesn't increase cache"
-        PID=$(cat /tmp/master/redis.pid)
+        PID=$(cat /var/tmp/master/redis.pid)
         kill -15 $PID
         while [ -x /proc/${PID} ]; do sleep 1; done
-        ./src/redis-server --daemonize yes --logfile /dev/null --dir /tmp/master --port 8080
+        ./src/redis-server --daemonize yes --logfile /dev/null --dir /var/tmp/master --port 8080
         
         # The daemonized parent can exit before the child listens, so tolerate
         # connection failures until INFO is available, then wait for RDB loading.
@@ -586,7 +586,7 @@ jobs:
         done
         sleep 1 # wait for the completion of cache reclaim bio
         
-        VMOUT=$(vmtouch -v /tmp/master/dump.rdb)
+        VMOUT=$(vmtouch -v /var/tmp/master/dump.rdb)
         echo $VMOUT
         grep -q " 0%" <<< $VMOUT
         CACHE=$(grep -w file /sys/fs/cgroup/memory.stat | awk '{print $2}')

src/util.c

@@ -1826,7 +1826,9 @@ static int cache_exist(int fd) {
 }
 
 static void test_reclaimFilePageCache(void) {
-    char *tmpfile = "/tmp/redis-reclaim-cache-test";
+    /* Use /var/tmp: /tmp may be tmpfs, where
+     * POSIX_FADV_DONTNEED does not evict file pages. */
+    char *tmpfile = "/var/tmp/redis-reclaim-cache-test";
     int fd = open(tmpfile, O_RDWR|O_CREAT, 0644);
     assert(fd >= 0);