Introduce RawStackTableBuilder for stack table construction.

`RawStackTable` is being prepared to allow `frame` to be an `Int32Array`
in a later commit. `Int32Array` is fixed-size and doesn't support
`push`, so the existing "push to .frame and bump .length" pattern needs
a builder that uses a plain `number[]` during construction and converts
to the final representation via `finishRawStackTableBuilder` at the end.

Switch all stack table construction sites to use the builder. The
builder still produces a plain `number[]` for `frame` in this commit;
the type change happens in a follow-up.

Author: mstange

src/profile-logic/data-structures.ts

@@ -27,6 +27,8 @@ import type {
   CallNodeTable,
   SourceTable,
   SourceLocationTable,
+  IndexIntoFrameTable,
+  IndexIntoStackTable,
 } from 'firefox-profiler/types';
 
 /**
@@ -47,7 +49,13 @@ export function getEmptySamplesTable(): RawSamplesTable {
   };
 }
 
-export function getEmptyRawStackTable(): RawStackTable {
+export type RawStackTableBuilder = {
+  frame: IndexIntoFrameTable[];
+  prefix: Array<IndexIntoStackTable | null>;
+  length: number;
+};
+
+export function getRawStackTableBuilder(): RawStackTableBuilder {
   return {
     // Important!
     // If modifying this structure, please update all callers of this function to ensure
@@ -59,6 +67,27 @@ export function getEmptyRawStackTable(): RawStackTable {
   };
 }
 
+export function getRawStackTableBuilderWithExistingContents(
+  existing: RawStackTable
+): RawStackTableBuilder {
+  return {
+    frame: [...existing.frame],
+    prefix: [...existing.prefix],
+    length: existing.length,
+  };
+}
+
+export function finishRawStackTableBuilder(
+  builder: RawStackTableBuilder
+): RawStackTable {
+  const { frame, prefix, length } = builder;
+  return {
+    frame,
+    prefix,
+    length,
+  };
+}
+
 /**
  * Returns an empty samples table with eventDelay field instead of responsiveness.
  * eventDelay is a new field and it replaced responsiveness. We should still
@@ -393,7 +422,7 @@ export function getEmptyThread(overrides?: Partial<RawThread>): RawThread {
 
 export function getEmptySharedData(): RawProfileSharedData {
   return {
-    stackTable: getEmptyRawStackTable(),
+    stackTable: finishRawStackTableBuilder(getRawStackTableBuilder()),
     frameTable: getEmptyFrameTable(),
     funcTable: getEmptyFuncTable(),
     resourceTable: getEmptyResourceTable(),

src/profile-logic/global-data-collector.ts

@@ -4,13 +4,14 @@
 
 import { StringTable } from '../utils/string-table';
 import {
+  finishRawStackTableBuilder,
   getEmptyFrameTable,
   getEmptyFuncTable,
   getEmptyNativeSymbolTable,
-  getEmptyRawStackTable,
   getEmptyResourceTable,
   getEmptySourceTable,
   getEmptySourceLocationTable,
+  getRawStackTableBuilder,
 } from './data-structures';
 
 import type {
@@ -22,7 +23,6 @@ import type {
   RawProfileSharedData,
   SourceTable,
   FrameTable,
-  RawStackTable,
   FuncTable,
   ResourceTable,
   NativeSymbolTable,
@@ -34,6 +34,7 @@ import type {
   Bytes,
 } from 'firefox-profiler/types';
 import { ResourceType } from 'firefox-profiler/types';
+import type { RawStackTableBuilder } from './data-structures';
 
 /**
  * GlobalDataCollector collects data which is global in the processed profile
@@ -50,7 +51,7 @@ export class GlobalDataCollector {
   _stringTable: StringTable = StringTable.withBackingArray(this._stringArray);
   _sources: SourceTable = getEmptySourceTable();
   _frameTable: FrameTable = getEmptyFrameTable();
-  _stackTable: RawStackTable = getEmptyRawStackTable();
+  _stackTableBuilder: RawStackTableBuilder = getRawStackTableBuilder();
   _funcTable: FuncTable = getEmptyFuncTable();
   _resourceTable: ResourceTable = getEmptyResourceTable();
   _nativeSymbols: NativeSymbolTable = getEmptyNativeSymbolTable();
@@ -302,15 +303,15 @@ export class GlobalDataCollector {
     return this._frameTable;
   }
 
-  getStackTable(): RawStackTable {
-    return this._stackTable;
+  getStackTableBuilder(): RawStackTableBuilder {
+    return this._stackTableBuilder;
   }
 
   // Package up all de-duplicated global tables so that they can be embedded in
   // the profile.
   finish(): { libs: Lib[]; shared: RawProfileSharedData } {
     const shared: RawProfileSharedData = {
-      stackTable: this._stackTable,
+      stackTable: finishRawStackTableBuilder(this._stackTableBuilder),
       frameTable: this._frameTable,
       funcTable: this._funcTable,
       resourceTable: this._resourceTable,

src/profile-logic/import/chrome.ts

@@ -501,7 +501,7 @@ async function processTracingEvents(
   const stringTable = globalDataCollector.getStringTable();
 
   const frameTable = globalDataCollector.getFrameTable();
-  const stackTable = globalDataCollector.getStackTable();
+  const stackTable = globalDataCollector.getStackTableBuilder();
 
   let profileEvents: (ProfileEvent | CpuProfileEvent)[] = (eventsByName.get(
     'Profile'

src/profile-logic/import/dhat.ts

@@ -183,7 +183,7 @@ export function attemptToConvertDhat(json: unknown): Profile | null {
   profile.meta.product = dhat.cmd + ' (dhat)';
   profile.meta.importedFrom = `dhat`;
   const globalDataCollector = new GlobalDataCollector();
-  const stackTable = globalDataCollector.getStackTable();
+  const stackTable = globalDataCollector.getStackTableBuilder();
   const frameTable = globalDataCollector.getFrameTable();
   const stringTable = globalDataCollector.getStringTable();
 

src/profile-logic/import/flame-graph.ts

@@ -60,7 +60,7 @@ export function convertFlameGraphProfile(profileText: string): Profile {
   });
 
   const frameTable = globalDataCollector.getFrameTable();
-  const stackTable = globalDataCollector.getStackTable();
+  const stackTable = globalDataCollector.getStackTableBuilder();
   const { samples } = thread;
 
   // Maps to deduplicate stacks, frames, and functions.

src/profile-logic/import/simpleperf.ts

@@ -24,7 +24,9 @@ import {
   getEmptyFuncTable,
   getEmptyResourceTable,
   getEmptyFrameTable,
-  getEmptyRawStackTable,
+  getRawStackTableBuilder,
+  finishRawStackTableBuilder,
+  type RawStackTableBuilder,
   getEmptySamplesTable,
   getEmptyRawMarkerTable,
   getEmptyNativeSymbolTable,
@@ -189,15 +191,15 @@ class FirefoxFrameTable {
 class FirefoxSampleTable {
   strings: StringTable;
 
-  stackTable: RawStackTable = getEmptyRawStackTable();
+  stackTable: RawStackTableBuilder = getRawStackTableBuilder();
   stackMap: Map<string, IndexIntoStackTable> = new Map();
 
   constructor(strings: StringTable) {
     this.strings = strings;
   }
 
   toJson(): RawStackTable {
-    return this.stackTable;
+    return finishRawStackTableBuilder(this.stackTable);
   }
 
   findOrAddStack(

src/profile-logic/js-tracer.ts

@@ -4,6 +4,8 @@
 import {
   getEmptySamplesTableWithEventDelay,
   getEmptyRawMarkerTable,
+  finishRawStackTableBuilder,
+  getRawStackTableBuilderWithExistingContents,
 } from './data-structures';
 import { StringTable } from '../utils/string-table';
 import { ensureExists } from '../utils/types';
@@ -512,7 +514,10 @@ export function convertJsTracerToThreadWithoutSamples(
     samples,
   };
 
-  const { funcTable, frameTable, stackTable } = shared;
+  const { funcTable, frameTable } = shared;
+  const stackTable = getRawStackTableBuilderWithExistingContents(
+    shared.stackTable
+  );
 
   // Keep a stack of js tracer events, and end timings, that will be used to find
   // the stack prefixes. Once a JS tracer event starts past another event end, the
@@ -620,6 +625,9 @@ export function convertJsTracerToThreadWithoutSamples(
     unmatchedEventEnds[unmatchedIndex] = end;
   }
 
+  // Write the augmented stackTable back to the shared data.
+  shared.stackTable = finishRawStackTableBuilder(stackTable);
+
   return { thread, stackMap };
 }
 

src/profile-logic/merge-compare.ts

@@ -13,7 +13,8 @@ import {
   getEmptyNativeSymbolTable,
   getEmptyFrameTable,
   getEmptyFuncTable,
-  getEmptyRawStackTable,
+  getRawStackTableBuilder,
+  finishRawStackTableBuilder,
   getEmptyRawMarkerTable,
   getEmptySamplesTableWithEventDelay,
   shallowCloneRawMarkerTable,
@@ -1117,7 +1118,7 @@ function mergeStackTables(
   translationMapsForFrames: TranslationMapForFrames[]
 ): { stackTable: RawStackTable; translationMaps: TranslationMapForStacks[] } {
   const translationMaps: TranslationMapForStacks[] = [];
-  const newStackTable = getEmptyRawStackTable();
+  const newStackTable = getRawStackTableBuilder();
 
   profiles.forEach((profile, profileIndex) => {
     const { stackTable } = profile.shared;
@@ -1143,7 +1144,10 @@ function mergeStackTables(
     translationMaps.push(oldStackToNewStackPlusOne);
   });
 
-  return { stackTable: newStackTable, translationMaps };
+  return {
+    stackTable: finishRawStackTableBuilder(newStackTable),
+    translationMaps,
+  };
 }
 
 /**

src/profile-logic/process-profile.ts

@@ -16,6 +16,7 @@ import {
   getEmptyRawMarkerTable,
   getEmptyJsAllocationsTable,
   getEmptyUnbalancedNativeAllocationsTable,
+  type RawStackTableBuilder,
 } from './data-structures';
 import { immutableUpdate, ensureExists } from '../utils/types';
 import { verifyMagic, SIMPLEPERF as SIMPLEPERF_MAGIC } from '../utils/magic';
@@ -53,7 +54,6 @@ import type {
   FrameTable,
   RawCounterSamplesTable,
   RawSamplesTable,
-  RawStackTable,
   RawMarkerTable,
   LibMapping,
   IndexIntoStackTable,
@@ -549,7 +549,7 @@ function _processFrameTable(
  */
 function _processStackTable(
   geckoStackTable: GeckoStackStruct,
-  sharedStackTable: RawStackTable,
+  sharedStackTable: RawStackTableBuilder,
   frameIndexOffset: IndexIntoFrameTable
 ): IndexIntoStackTable {
   const stackIndexOffset = sharedStackTable.length;
@@ -1337,7 +1337,7 @@ function _processThread(
   );
   const stackIndexOffset = _processStackTable(
     geckoStackTable,
-    globalDataCollector.getStackTable(),
+    globalDataCollector.getStackTableBuilder(),
     frameIndexOffset
   );
 

src/profile-logic/profile-data.ts

@@ -6,7 +6,8 @@ import memoize from 'memoize-immutable';
 import MixedTupleMap from 'mixedtuplemap';
 import { oneLine } from 'common-tags';
 import {
-  getEmptyRawStackTable,
+  getRawStackTableBuilder,
+  finishRawStackTableBuilder,
   getEmptyCallNodeTable,
   shallowCloneFrameTable,
   shallowCloneFuncTable,
@@ -4343,7 +4344,7 @@ export function nudgeReturnAddresses(profile: Profile): Profile {
   // Now the frame table contains adjusted / "nudged" addresses.
 
   // Make a new stack table which refers to the adjusted frames.
-  const newStackTable = getEmptyRawStackTable();
+  const newStackTable = getRawStackTableBuilder();
   const mapForSamplingSelfStacks = new Map<
     null | IndexIntoStackTable,
     null | IndexIntoStackTable
@@ -4385,7 +4386,7 @@ export function nudgeReturnAddresses(profile: Profile): Profile {
   const newShared: RawProfileSharedData = {
     ...profile.shared,
     frameTable: newFrameTable,
-    stackTable: newStackTable,
+    stackTable: finishRawStackTableBuilder(newStackTable),
   };
 
   const newThreads = updateRawThreadStacksSeparate(