Getting StringIndexOutOfBoundsException on textDocument/signatureHelp request when triggered from the end of document

[ajaydeopa]

Issue

When we trigger a textDocument/signatureHelp request from the end of the document, it throws java.lang.StringIndexOutOfBoundsException as the offset exceeds the buffer length.

How to Reproduce

Trigger a textDocument/signatureHelp request from the end of the document (ensure to write both open & close parenthesis manually).

Stack trace

java.util.concurrent.CompletionException: java.lang.StringIndexOutOfBoundsException: String index out of range: 6
  at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:315)
  at java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:320)
  at java.base/java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:649)
  at java.base/java.util.concurrent.CompletableFuture$Completion.exec(CompletableFuture.java:483)
  at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373)
  at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1182)
  at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1655)
  at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1622)
  at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:165)
Caused by: java.lang.StringIndexOutOfBoundsException: String index out of range: 6
  at java.base/java.lang.StringLatin1.charAt(StringLatin1.java:48)
  at java.base/java.lang.String.charAt(String.java:1513)
  at org.eclipse.jdt.ls.core.internal.handlers.SignatureHelpContext.findTargetNode(SignatureHelpContext.java:132)
  at org.eclipse.jdt.ls.core.internal.handlers.SignatureHelpContext.resolve(SignatureHelpContext.java:103)
  at org.eclipse.jdt.ls.core.internal.handlers.SignatureHelpUtils.getSignatureHelpFromASTNode(SignatureHelpUtils.java:52)
  at org.eclipse.jdt.ls.core.internal.handlers.SignatureHelpHandler.signatureHelp(SignatureHelpHandler.java:79)
  at org.eclipse.jdt.ls.core.internal.handlers.JDTLanguageServer.lambda$8(JDTLanguageServer.java:649)
  at org.eclipse.jdt.ls.core.internal.BaseJDTLanguageServer.lambda$0(BaseJDTLanguageServer.java:87)
  at java.base/java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:646)
... 6 more