Truncate container log files to remove logging delay on restarted container (#26)

When a container that was writing stdout/stderr to its dedicated
container log file was stopped and started, there was a ~20 second - 1
minute delay from when the init process was restarted and when the
output of the process was written to the log file. This also meant there
was a delay when streaming those logs using `container logs -f`. The
logs from that delay period were lost.

This change forces the container log file to update state after a
container restart which fixes the delays to the container log file. And
the` container logs -f` stream resets its position in the file on a
container restart as well.

---------

Co-authored-by: Arnav Reddy <areddy23@apple.com>

Sources/CLI/Container/ContainerLogs.swift

@@ -117,9 +117,14 @@ extension Application {
                 fh.readabilityHandler = { handle in
                     let data = handle.availableData
                     if data.isEmpty {
-                        fh.readabilityHandler = nil
-                        cont.finish()
-                        return
+                        // Triggers on container restart - can exit here as well
+                        do {
+                            _ = try fh.seekToEnd()  // To continue streaming existing truncated log files
+                        } catch {
+                            fh.readabilityHandler = nil
+                            cont.finish()
+                            return
+                        }
                     }
                     if let str = String(data: data, encoding: .utf8), !str.isEmpty {
                         var lines = str.components(separatedBy: .newlines)

Sources/Services/ContainerSandboxService/SandboxService.swift

@@ -697,7 +697,8 @@ extension ContainerClient.Bundle {
 
     func createLogFile() throws {
         // Create the log file we'll write stdio to.
-        let fd = Darwin.open(self.containerLog.path, O_CREAT | O_RDONLY, 0o644)
+        // O_TRUNC resolves a log delay issue on restarted containers by force-updating internal state
+        let fd = Darwin.open(self.containerLog.path, O_CREAT | O_RDONLY | O_TRUNC, 0o644)
         guard fd > 0 else {
             throw POSIXError(.init(rawValue: errno)!)
         }