Fix PerfMap crash when Enable IPC command is sent early in startup

[Copilot]

Description

Fixes #123438

PerfMap::Enable(sendExisting=true) crashes when a PerfMapEnable IPC command is received very early in startup (e.g., during PauseForDiagnosticsMonitor). At this point:

• GetAppDomain() returns NULL (created in SystemDomain::Attach)
• ExecutionManager::GetEEJitManager() returns NULL (created in ExecutionManager::Init)

Both are initialized after DiagnosticServerAdapter::Initialize, so IPC commands can arrive before these are ready.

This PR adds a volatile bool to signal when sendExisting logic dependencies are initialized, otherwise sendExisting logic is skipped. sendExisting logic is safe to skip when called early - no assemblies are loaded and no code is JIT'd at that point. Future JIT'd methods will still be logged.

Startup Timeline

PerfMap::Initialize()
DiagnosticServerAdapter::Initialize()    ← IPC listener starts
PauseForDiagnosticsMonitor()             ← Can receive PerfMapEnable here
...
SystemDomain::Attach()                   ← AppDomain created
ExecutionManager::Init()                 ← EEJitManager created
PerfMap::SignalSendExistingReady()       ← Flag set, sendExisting now safe

Testing

mdh1418 validated on a wsl2 instance, pausing the startup logic with DOTNET_DefaultDiagnosticPortSuspend=1 and kicking off a separate app invoking DiagnosticClient.EnablePerfMap. Before the change, it would sigsegv and crash.
After the changes, its resilient and doesn't crash.

Original prompt

Can you investigate ways to fix #123438

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[Copilot]

Pull request overview

Fixes a CoreCLR startup crash when a PerfMapEnable IPC command arrives very early (before AppDomain and/or EEJitManager are initialized), by making PerfMap::Enable(sendExisting: true) resilient to those components being unavailable.

Changes:

• Guard assembly enumeration behind a null-check for GetAppDomain() to avoid calling IterateAssembliesEx before SystemDomain::Attach().
• Guard JIT code heap enumeration behind a null-check for ExecutionManager::GetEEJitManager() to avoid iterating heaps before ExecutionManager::Init().
• Reuse the EEJitManager* local when building the CodeHeapIterator.

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[mdh1418]

/backport to release/10.0

[github-actions]

Started backporting to release/10.0 (link to workflow run)

[github-actions]

@mdh1418 backporting to release/10.0 failed, the patch most likely resulted in conflicts. Please backport manually!

git am output

$ git am --3way --empty=keep --ignore-whitespace --keep-non-patch changes.patch

Applying: Add null check for AppDomain in early-startup PerfMap::Enable
Applying: Add null check for EEJitManager in early-startup PerfMap::Enable
Using index info to reconstruct a base tree...
M	src/coreclr/vm/perfmap.cpp
Falling back to patching base and 3-way merge...
Auto-merging src/coreclr/vm/perfmap.cpp
CONFLICT (content): Merge conflict in src/coreclr/vm/perfmap.cpp
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config set advice.mergeConflict false"
Patch failed at 0002 Add null check for EEJitManager in early-startup PerfMap::Enable
Error: The process '/usr/bin/git' failed with exit code 128

Link to workflow output