RFR: 8371309: Diagnostic.getEndPosition can throw an NPE with typical broken code [v2]

[Jan Lahoda]

> When starting the compiler via API, a `DiagnosticListener` may be provided, to which all reported errors and warnings are reported.
> 
> The reported diagnostics are represented by `javax.tools.Diagnostic`, which has (among others) method `getEndPosition`, which requires the end-pos-table to be set in the `DiagnosticSource` object for the given file (see `AbstractLog.sourceMap`). But the end-pos-table is only set after the parse is done:
> https://github.com/openjdk/jdk/blob/167c952bb0fefb5acc9782f4f4474d92097c93f8/src/jdk.compiler/share/classes/com/sun/tools/javac/main/JavaCompiler.java#L702
> 
> There are three problems with this:
> - when a `Diagnostic` is reported by the parser, and the clients code calls `Diagnostic.getEndPosition`, the end-pos-table is not set yet, and the method crashes with an NPE
> - the end-pos-table is never set for implicitly compiled (parsed) files:
> https://github.com/openjdk/jdk/blob/167c952bb0fefb5acc9782f4f4474d92097c93f8/src/jdk.compiler/share/classes/com/sun/tools/javac/main/JavaCompiler.java#L840
> - when a `Diagnostic` is reported for a tree by the parser, the tree on which the `Diagnostic` appear does not typically have the end position set yet - it is usually set only after the error/warning is reported
> 
> All those problems used to be hidden by this check:
> https://github.com/openjdk/jdk/blob/6c48f4ed707bf0b15f9b6098de30db8aae6fa40f/src/jdk.compiler/share/classes/com/sun/tools/javac/tree/TreeInfo.java#L649
> but that has been removed:
> https://github.com/openjdk/jdk/commit/c793de989facdb532021e1d5ddd01eb0e089b8e6
> 
> I have tried to fix this properly, by deferring the errors from the parser to a later point, properly setting the end-pos-table, and handling implicitly parsed files more correctly:
> https://github.com/openjdk/jdk/compare/master...lahodaj:jdk:JDK-8371309b?expand=1
> but this change is a bit involved, and potentially a bit dangerous.
> 
> So, for now, I propose to reinstate the null check in `TreeInfo.getEndPos`, to restore the JDK 25 behavior, and attempt to do the better solution later, hopefully earlier in the release cycle.

Jan Lahoda has updated the pull request incrementally with one additional commit since the last revision:

  Never return NOPOS from (JCTree-based) Diagnostic's start and end position.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/28316/files
  - new: https://git.openjdk.org/jdk/pull/28316/files/ef1f68a7..384fed72

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28316&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28316&range=00-01

  Stats: 37 lines in 2 files changed: 35 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/28316.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28316/head:pull/28316

PR: https://git.openjdk.org/jdk/pull/28316