8356165: System.in in jshell replace supplementary characters with ??

[lahodaj]

When reading from System.in in a JShell snippet, JShell first reads the whole line (getting a String), and then converts this characters from this String to bytes on demand. But, it does not convert multi-surrogate code points correctly, it tries to convert each surrogate separately, which cannot work.

The proposal herein is to, when the current character is a high surrogate, peek at the next character, and if it is a low surrogate, convert both the high and low surrogates to bytes together.

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Issue

• JDK-8356165: System.in in jshell replace supplementary characters with ?? (Bug - P3)

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[mlbridge]

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• 00: Full (42052fcb)

[sormuras]

Perhaps, add here the comment from the PR description for readers from the future:

[...] when the current character is a high surrogate, peek at the next character, and if it is a low surrogate, convert both the high and low surrogates to bytes together.

The (internal) API used in the implementation doesn't express that on first sight.

[tats-u]

How about combining readUserInputChar() and (only when not surrogate pair but just isolated code unit) pendingLinePointer--?

pendingLinePointer-- will be unlikely to be happen for normal inputs other than penetration tests.

[tats-u]

I think the following is robuster:

-            inputSink.write("new String(System.in.readNBytes(4))\n\uD83D\uDE03\n");
-            waitOutput(out, "\"\uD83D\uDE03\"");
+            inputSink.write("new String(System.in.readNBytes(5))\n\uD83D\uDE031\n");
+            waitOutput(out, "\"\uD83D\uDE031\"");