Fix crash when importing big linux perf tool files

Currently, importing files generated by linux perf tool whose some blocks exceed V8 strings limit can crash the application. This issue is similar to the one in #385.

This PR fixes it by changing parseEvents to work directly with lines instead of chunking lines into blocks first.

Fixes #433

src/import/linux-tools-perf.ts

@@ -23,8 +23,23 @@ interface PerfEvent {
   stack: PerfStackFrame[]
 }
 
-function parseEvent(rawEvent: string): PerfEvent | null {
-  const lines = rawEvent.split('\n').filter(l => !/^\s*#/.exec(l))
+function parseEvents(contents: TextFileContent) {
+  let events = []
+  let buffer = []
+  for (let line of contents.splitLines()) {
+    if (line === '') {
+      events.push(parseEvent(buffer))
+      buffer = []
+    } else buffer.push(line)
+  }
+
+  if (buffer.length > 0) events.push(parseEvent(buffer))
+  return events
+}
+
+// rawEvent is splitted into lines
+function parseEvent(rawEvent: string[]): PerfEvent | null {
+  const lines = rawEvent.filter(l => !/^\s*#/.exec(l))
 
   const event: PerfEvent = {
     command: null,
@@ -81,41 +96,12 @@ function parseEvent(rawEvent: string): PerfEvent | null {
   return event
 }
 
-function splitBlocks(contents: TextFileContent): string[] {
-  // In perf files, blocks are separated by '\n\n'.  If our input was a simple
-  // string, we could use str.split('\n\n'), but since we have a TextFileContent
-  // object which may be backed by several strings, we can't easily split this
-  // way.
-  //
-  // Instead, we'll split into lines, and then re-group runs of non-empty strings.
-  const blocks: string[] = []
-  let pending: string = ''
-  for (let line of contents.splitLines()) {
-    if (line === '') {
-      if (pending.length > 0) {
-        blocks.push(pending)
-      }
-      pending = line
-    } else {
-      if (pending.length > 0) {
-        pending += '\n'
-      }
-      pending += line
-    }
-  }
-  if (pending.length > 0) {
-    blocks.push(pending)
-  }
-  return blocks
-}
-
 export function importFromLinuxPerf(contents: TextFileContent): ProfileGroup | null {
   const profiles = new Map<string, StackListProfileBuilder>()
 
   let eventType: string | null = null
-  const events = splitBlocks(contents).map(parseEvent)
 
-  for (let event of events) {
+  for (let event of parseEvents(contents)) {
     if (event == null) continue
     if (eventType != null && eventType != event.eventType) continue
     if (event.time == null) continue