IL: map the short-lived metadata-only PE reader - #20489
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This view exists only to parse the PE headers and copy the resources out, and is disposed immediately after, so reading the whole assembly into a managed byte array materialises megabytes to touch a few pages. Mapping it is safe because neither pectxtEager nor pevEager may be captured by the results. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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The cleanup lived in an explicit IDisposable implementation, which Stream.Dispose() never reaches, so getBinaryFile's disposer left the file mapped until the safe handles were finalized. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
T-Gro
reviewed
Sep 9, 2026
| // For metadata-only, always use a temporary, short-lived PE file reader, preferably over a memory mapped file. | ||
| // Then use the metadata blob as the long-lived memory resource. | ||
| let disposer, pefileEager = getBinaryFile fullPath false | ||
| let disposer, pefileEager = getBinaryFile fullPath true |
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🤖🕵️ [P2] Empty references leave a read handle open until GC, blocking an exclusive reopen after FCS reports the invalid assembly. CreateFromFile throws before disposer exists; the mapping-construction failure path needs to dispose its opened FileStream.
open System
open System.IO
open FSharp.Compiler.CodeAnalysis
open FSharp.Compiler.Text
let dll = Path.GetFullPath "empty.dll"
let script = Path.GetFullPath "check.fsx"
File.WriteAllBytes(dll, [||])
let source = SourceText.ofString (sprintf "#r @\"%s\"\nlet value = 1" dll)
let checker = FSharpChecker.Create()
let options, _ =
checker.GetProjectOptionsFromScript(script, source, useSdkRefs = true)
|> Async.RunSynchronously
// Keep GC from hiding the leaked handle during this repro.
if GC.TryStartNoGCRegion(256L * 1024L * 1024L) then
try
checker.ParseAndCheckFileInProject(script, 0, source, options)
|> Async.RunSynchronously |> ignore
use retry = new FileStream(dll, FileMode.Open, FileAccess.ReadWrite, FileShare.None)
() // Base succeeds; mapped path throws IOException on Windows.
finally
GC.EndNoGCRegion()
T-Gro
self-requested a review
September 9, 2026 08:58
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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OpenILModuleReader's metadata-only branch — the FCS/IDE path,reduceMemoryUsage = Yes,metadataOnly = Yes— opens a temporary PE reader to parse the headers, locate the metadata directory andcopy the resources out, then throws it away:
Allocation
Time
Warm whole-solution, 10 in-process passes