memory-monitor: Improve resilience of handler to missing processes.

Enhance the script to handle missing processes more gracefully in the
memory event handler. Instead of halting execution when a process is no
longer present, the handler now skips over non-existent processes. This
allows the memory event handling to continue smoothly when triggered by
the monitor, even if some processes have terminated. The added checks
ensure robust handling of ps and /proc access failures.

Signed-off-by: Nikolay Martyanov <nikolay@zededa.com>
(cherry picked from commit dd27fe1)

pkg/memory-monitor/src/monitor/memory-monitor-handler.sh

@@ -88,10 +88,21 @@ normalize_pids() {
     sorted_processes_file=$2
     sort -u "$processes_file" -o "$processes_file"
     while read -r pid; do
-       ps -p "$pid" -o pid=,rss=
+      # Check if the process still exists, skip if it does not
+      if ! ps -p "$pid" -o pid=,rss= ; then
+        continue
+      fi
     done < "$processes_file" | sort -k2,2 -n -r | awk '{print $1}' > "$sorted_processes_file"
 }
 
+print_process_gone() {
+  detailed=$1
+  if [ "$detailed" -eq 1 ]; then
+    echo "  process is gone during the handling, ignore it" >> "$output_file"
+    echo "" >> "$output_file"
+  fi
+}
+
 show_pid_mem_usage() {
   cgroup_name=$1
   processes_file=$2
@@ -117,16 +128,31 @@ show_pid_mem_usage() {
       # Process is gone
       continue
     fi
-    name=$(ps -p "$pid" -o cmd=)
+    if ! name=$(ps -p "$pid" -o cmd=) ; then
+      # Process is gone. We have not yet printed any information about the process
+      # so we can skip it without any explicit message in the output
+      continue
+    fi
     if [ "$detailed" -eq 1 ]; then
       echo "Process $name PID: $pid" >> "$output_file"
+      # Since now we have printed the process header in the detailed view, hence
+      # if the process is gone later, we should reflect that in the output
     fi
     # Get the memory usage according to ps
-    ps_rss=$(ps -p "$pid" -o rss= | tr -d ' ')
+    if ! ps_rss=$(ps -p "$pid" -o rss= | tr -d ' ') ; then
+      # Process is gone
+      print_process_gone "$detailed"
+      continue
+    fi
     # Read the smaps file line by line
     rss_pid=0
     tmp_smaps=$(mktemp)
-    cat /proc/"$pid"/smaps > "$tmp_smaps"
+    if ! cat /proc/"$pid"/smaps > "$tmp_smaps" ; then
+      # Process is gone
+      rm "$tmp_smaps"
+      print_process_gone "$detailed"
+      continue
+    fi
     while read -r line; do
       # Check for lines containing 'Pss:'
       # shellcheck disable=SC3010 # we use the busybox version of sh, which does support the [[ operator