Fix array parsing across content streams (cherry-pick from upstream) - #9
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ChangesPDF content-stream parsing
Priority: ➖ Normal Estimated code review effort: 3 (Moderate) | ~20 minutes Change: Bug fix Sequence Diagram(s)sequenceDiagram
participant Interpret
participant Buffer
participant Context
Interpret->>Context: Check cancellation between tokens
Interpret->>Buffer: Read next token
Buffer->>Context: Check cancellation before stream lookup
Buffer-->>Interpret: Return token or context error
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readToken returns io.EOF as a token value once the input is exhausted,
but readArray only broke out of its loop on nil or keyword("]"). On a
PDF whose content stream is truncated inside an unterminated array, the
loop unread the io.EOF token, read it back as an object, and appended
it to the array forever, allocating memory without bound (~7GB of heap
in 5 seconds observed on a real-world malformed PDF) and hanging every
text-extraction entry point (GetPlainText, Page.Content,
Page.GetTextByRow).
readDict already guards against io.EOF; give readArray the same guard.
Add a regression test that synthesizes a minimal single-page PDF whose
content stream ends inside an unterminated array and verifies that
GetPlainText returns instead of spinning. The test times out against
the previous code and passes with the fix.
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Inline comments:
In `@lex_test.go`:
- Line 61: Update the test using GetPlainText to capture and assert that it
returns no error, while preserving the existing termination assertions so
earlier parsing failures cannot be mistaken for normal io.EOF completion.
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lex.golex_test.go
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Inline comments:
In `@ps.go`:
- Line 68: Update the reader assembly around io.MultiReader to insert a single
whitespace reader between every adjacent content stream, while preserving each
stream’s original bytes and order. Add a regression case covering adjacent
streams with no boundary whitespace, such as “10” and “20 m”, and verify they
are separated correctly.
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ps.gops_test.go
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Interpret concatenated an array /Contents via io.MultiReader without any
separator between streams. Per the PDF spec, streams in a content-stream
array must be treated as if joined with a space, since a token can't span
two streams; a stream ending in a partial numeric token immediately
followed by another stream starting with digits (e.g. "10" then "20 m")
would otherwise merge into a single token ("1020").
| // merge into "1020"). | ||
| readers = append(readers, strings.NewReader(" ")) | ||
| } | ||
| readers = append(readers, strm.Index(i).Reader()) |
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Also from upstream, but we make it a bit worse I think -- this eagerly creates every stream reader before parsing starts or the context is checked. Reader() initializes filters immediately, so repeated filtered stream references can retain roughly 2 MiB of predictor buffers per entry and will OOM even for an already-canceled request. The eager construction comes from upstream. But now we also bypasses the cancellation guarantee added to Interpret.
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Thanks! The new check catches an already-canceled context, but with a live request we still build and retain every reader before parsing starts. iiuc, repeated predictor streams can pile up about 2 MiB each before the first token or output-limit cancellation. Also, the new test only covers one stream with the context canceled before Interpret starts, so it misses this case.
An empty /Contents array made the reader-slice capacity 2*0-1 == -1, which panics in make(). Clamp the capacity to zero.
strm.Index(i).Reader() runs that stream's decode filters immediately (e.g. Predictor allocates ~2MiB), and it ran for every stream up front regardless of cancellation. Check ctx.Done() before each one so a canceled context stops before paying that cost.
…on test TestUnterminatedArrayTerminates discarded GetPlainText's return values, so an unrelated parse error could pass as normal EOF termination. Also used context.Background(), so on a regression the background goroutine would keep running past the test's own 5s failsafe instead of being cancelled. Capture the error and use context.WithTimeout so cancellation actually stops the goroutine on a regression, and keep the deadline (2s) shorter than the failsafe (5s) so cancellation reliably wins the race and the failure reports the real cause instead of a generic timeout.
| // merge into "1020"). | ||
| readers = append(readers, strings.NewReader(" ")) | ||
| } | ||
| readers = append(readers, strm.Index(i).Reader()) |
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Thanks! The new check catches an already-canceled context, but with a live request we still build and retain every reader before parsing starts. iiuc, repeated predictor streams can pile up about 2 MiB each before the first token or output-limit cancellation. Also, the new test only covers one stream with the context canceled before Interpret starts, so it misses this case.
…daries Interpret built every stream's reader up front, so a live request with repeated predictor streams allocated ~2 MiB per stream before the first token, and output-limit cancellation could never fire in time. It also joined the streams with a space, which let comments, strings and unclosed arrays at the end of one stream swallow the next one. The buffer now holds the /Contents array and nextStream switches to the next stream only when the current one hits EOF, so at most one decoder is alive at a time. Each stream ends in its own EOF, so no token spans two streams as the PDF spec requires; the Interpret loop, readArray and readDict continue into the next stream so split operands such as TJ arrays still parse. readArray ends an unclosed array at the first operator instead of panicking. Tests share one PDF builder and cover lazy opening, pre-canceled contexts and malformed stream boundaries. Three tests fail on purpose, and on master as well, to track pre-existing bugs: null or dangling /Contents entries, reload panicking on cancel mid-token, and the unterminated hex string hang.
…cked bugs nextStream is now called only from readToken, between tokens, so every reader above it (operands, arrays, dict keys and values) continues into the next /Contents stream while each token still ends at its own stream's EOF. A dict split between a key and its value now parses as one dict. It also skips array entries that aren't streams: null entries and references to missing objects resolve to null, which the spec treats as absent content. Interpret returns a cancellation that the lexer raises mid-token as ctx.Err() instead of panicking, and readCmap drops a partially built cmap when Interpret returns an error. readHexString stops at EOF instead of skipping the whitespace readByte reports there forever. The three tests that tracked these bugs now pass, and a new test covers dicts split across streams. Cleanup: remove the unused seqReader and a commented-out panic, replace the loop's ctx select with ctx.Err(), move the keyword switch's default case last, and reduce TestUnterminatedArrayTerminates to a plainText call.
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heh, one more, sorry Filipe :)
| // readToken calls it only between tokens, so tokens stop at their stream's EOF | ||
| // while operands, arrays and dicts continue; one decoder is alive at a time. | ||
| func (b *buffer) nextStream() bool { | ||
| for b.streamIdx < b.streams.Len() { |
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Sorry Filipe , looks like there's another small issue because of our adding of context: upstream already walks every /Contents entry, but our Interpret has a cancellable context. This loop can resolve a long run of indirect null entries without observing that context, reparsing each reference along the way. A crafted PDF can keep extraction busy after the caller cancels. We did have a per-entry context check before -- we should keep it, I think
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Summary
Cherry-picked from upstream
ledongthuc/pdf, which already fixed the same issue this PR was originally targeting (MM-70600):601f614—readArraynow breaks onio.EOF(previously only broke onnil/], matchingreadDict's existing guard).8b43568—Interpretnow reads an array/Contentsas a single concatenated stream viaio.MultiReader, instead of lexing each stream separately, so tokens (e.g. an array literal forTJ) split across streams parse correctly.The second commit conflicted with our
ps.go(MM-69725 addedcontext.Contextcancellation toInterpret). Resolved by keeping upstream's single-reader restructuring while preserving ourb.ctxwiring and<-ctx.Done()poll.Both commits' test files (
lex_test.go,ps_test.go) were adapted to compile against this fork'sGetPlainText(ctx)signature, which doesn't exist upstream.Picking the upstream commits as-is keeps this fork compatible with future upstream merges instead of diverging with a locally-authored fix.
Test plan
go build ./...go vet ./...go test ./...