procdockerstatsd-rs: use ASCII hyphen for invalid container name (#346)

Replace Unicode dash with ASCII hyphen-minus in INVALID_CONTAINER_NAME constant and related comment, so the invalid container name matches what docker stats actually returns.

Why I did this
We found some error in syslog:

ERR memory_threshold_check: Failed to parse memory usage for "{'CPU%': '--', 'MEM%': '--', 'MEM_BYTES': '0', 'MEM_LIMIT_BYTES': '0', 'NAME': '--', 'PIDS': '--'}": could not convert string to float: '--'
2026 Feb  6 22:43:04.172360 air-2700-1 ERR memory_threshold_check: Failed to parse memory usage for "{'CPU%': '--', 'MEM%': '--', 'MEM_BYTES': '0', 'MEM_LIMIT_BYTES': '0', 'NAME': '--', 'PIDS': '--'}": could not convert string to float: '--'
2026 Feb  6 22:43:04.172422 air-2700-1 ERR memory_threshold_check: Failure occurred could not convert string to float: '--'
The error is statistical. The flow is as below:
procdockerstatsd calls "docker stats" command periodically, parse the output and store to STATE DB
memory_threshold_check which is called by monit service, handles the data in STATE DB and check the memory usage

For detailed docker code, please check:
https://github.com/docker/cli/blob/93fa57bbcd08f2f5be7f6cf22f4273a2b5a49e71/cli/command/container/formatter_stats.go#L25
https://github.com/docker/cli/blob/93fa57bbcd08f2f5be7f6cf22f4273a2b5a49e71/cli/command/container/formatter_stats.go#L169

How I did this
procdockerstatsd should check the command output and ignore invalid value:
If the name is empty of "--", ignore the output, log a warning message to syslog.

How I test this
Manual test.

Author: jianyuewu

crates/procdockerstatsd-rs/src/main.rs

@@ -15,7 +15,7 @@ use std::ffi::CString;
 use serde::Deserialize;
 
 const UPDATE_INTERVAL: u64 = 120; // 2 minutes
-const INVALID_CONTAINER_NAME: &str = "—-"; // invalid container name returned by docker stats command
+const INVALID_CONTAINER_NAME: &str = "--"; // invalid container name returned by docker stats command
 
 #[derive(Debug, Deserialize)]
 #[serde(rename_all = "PascalCase")]
@@ -106,7 +106,7 @@ fn parse_docker_json_output(json_output: &str) -> HashMap<String, HashMap<String
 
         if stats.name.is_empty() || stats.name == INVALID_CONTAINER_NAME {
             // If a container stops suddenly after we send the docker stats command,
-            // it might return with a container name "—-". We should ignore such output.
+            // it might return with a container name "--". We should ignore such output.
             warn!("Skipping docker stats JSON for container {} with output: {}", stats.id, line);
             continue;
         }
@@ -387,3 +387,29 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     daemon.run();
     Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Invalid container line: NAME "--", other fields "--" or "0", no ID (empty).
+    /// Matches real docker stats output that leads to "Failed to parse memory usage" when not skipped.
+    const INVALID_LINE: &str = r#"{"ID":"","Name":"--","CPUPerc":"--","MemPerc":"--","MemUsage":"--","NetIO":"--","BlockIO":"--","PIDs":"--"}"#;
+
+    /// Valid container line with proper MemUsage/NetIO/BlockIO format.
+    const VALID_LINE: &str = r#"{"ID":"abc123","Name":"valid-container","CPUPerc":"0.5%","MemPerc":"1.2%","MemUsage":"100MiB / 2GiB","NetIO":"1kB / 2kB","BlockIO":"3MB / 4MB","PIDs":"10"}"#;
+
+    #[test]
+    fn test_parse_docker_json_skips_invalid_container_name() {
+        let result = parse_docker_json_output(INVALID_LINE);
+        assert!(result.is_empty(), "lines with NAME \"--\" (invalid container) should be skipped");
+    }
+
+    #[test]
+    fn test_parse_docker_json_includes_valid_container() {
+        let result = parse_docker_json_output(VALID_LINE);
+        assert_eq!(result.len(), 1);
+        assert!(result.contains_key("DOCKER_STATS|abc123"));
+        assert_eq!(result["DOCKER_STATS|abc123"].get("NAME").map(String::as_str), Some("valid-container"));
+    }
+}