Make --max-samples stop precisely at the limit

Previously, the sample limit was only checked at the start of each main
loop iteration, allowing multiple samples to be processed before the
check. Now the limit is checked inside the parse() loop before each
message is processed, stopping immediately when the limit is reached.

Changes:
- proto.d: Add wantData() check at start of parse loop, return 0 when done
- subproc.d: Add sampleLimit pointer and wantData() method
- main.d: Set sample limit pointer on subprocesses

No new global state - the limit is a local variable with all subprocesses
pointing to it. Uses ulong.max to represent "no limit".

Author: CyberShadow

source/btdu/main.d

@@ -139,10 +139,13 @@ Please report defects and enhancement requests to the GitHub issue tracker:
 	if (maxTime)
 		parsedMaxTime = parseDuration(maxTime);
 
-	ulong parsedMaxSamples;
+	ulong parsedMaxSamples = ulong.max;  // ulong.max means no limit
 	if (maxSamples)
 		parsedMaxSamples = maxSamples.to!ulong;
 
+	foreach (ref subproc; subprocesses)
+		subproc.sampleLimit = &parsedMaxSamples;
+
 	@property real parsedMinResolution()
 	{
 		static Nullable!real value;
@@ -244,6 +247,7 @@ Please report defects and enhancement requests to the GitHub issue tracker:
 				}
 				// Only pause once
 				maxSamples = maxTime = minResolution = null;
+				parsedMaxSamples = ulong.max;
 			}
 		}
 

source/btdu/proto.d

@@ -216,6 +216,9 @@ size_t parse(H)(ref ubyte[] buf, ref H handler)
 {
 	while (true)
 	{
+		if (!handler.wantData())
+			return 0;
+
 		if (buf.length < Header.sizeof)
 			return Header.sizeof - buf.length;
 

source/btdu/subproc.d

@@ -50,6 +50,7 @@ struct Subprocess
 	Pipe pipe;
 	Socket socket;
 	Pid pid;
+	ulong* sampleLimit;  /// Points to limit value; ulong.max means no limit
 
 	void start()
 	{
@@ -81,6 +82,12 @@ struct Subprocess
 	/// Section of buffer containing received and unparsed data
 	private size_t bufStart, bufEnd;
 
+	/// Check if more data is wanted (called by proto.parse before each message)
+	bool wantData()
+	{
+		return browserRoot.getSamples(SampleType.represented) < *sampleLimit;
+	}
+
 	/// Called when select() identifies that the process wrote something.
 	/// Reads one datum; returns `true` if there is more to read.
 	bool handleInput()
@@ -92,6 +99,10 @@ struct Subprocess
 		bufStart = bufEnd - data.length;
 		if (bufStart == bufEnd)
 			bufStart = bufEnd = 0;
+
+		if (bytesNeeded == 0)
+			return false;
+
 		if (buf.length < bufEnd + bytesNeeded)
 		{
 			// Moving remaining data to the start of the buffer