ICU-20558 Fix compat issue with DateTimePatternGenerator fallback to default root locale

[jefgen]

(Thanks to @gvictor for tracking down the issue.)

This commit for ICU-13778 improved the error checking in the DateTimePatternGenerator class, adding checks for errors/failures where there previously was none at all. This was done in order to catch catastrophic errors like out-of-memory (OOM), and properly report them to the caller, rather than ignoring/hiding these errors.

However, in doing so it exposed a case where the code was depending on ignoring errors in order to fallback to the Gregorian calendar when the default ICU locale is set to root.

This commit restores the previous behavior, by allowing the error of U_MISSING_RESOURCE_ERROR to fall-though and continue without reporting back an error.

Note: This regression was introduced in ICU 63, and also effects ICU 64 as well.

Checklist

• Issue filed: https://unicode-org.atlassian.net/browse/ICU-20558
• Updated PR title and link in previous line to include Issue number
• Issue accepted
• Tests included
• Documentation is changed or added

[jefgen]

FYI for @tarekgh.

[tarekgh]

Thanks @jefgen for tagging me to the issue. could you please confirm this fix will fix the scenario mentioned in https://unicode-org.atlassian.net/browse/ICU-20558 when usign LANG=C.UTF-8?

[gvictor]

LGTM, but it's a nit comment due to my unfamiliarity with code style

[jefgen]

could you please confirm this fix will fix the scenario mentioned in https://unicode-org.atlassian.net/browse/ICU-20558 when using LANG=C.UTF-8?

FYI: @tarekgh with the change in this PR, the following code snippet now succeeds with U_USING_DEFAULT_WARNING, and no longer returns U_MISSING_RESOURCE_ERROR.

#include <stdio.h>
#include "unicode/udatpg.h"

int main()
{
  UErrorCode er = U_ZERO_ERROR;
  UDateTimePatternGenerator* pGenerator = udatpg_open("C", &er);

  UChar monthDayPattern[100];
  const UChar UDAT_MONTH_DAY_UCHAR[] = {'M', 'M', 'M', 'M', 'd', '\0'};

  udatpg_getBestPattern(pGenerator, UDAT_MONTH_DAY_UCHAR, -1, monthDayPattern, 100, &er);
  udatpg_close(pGenerator);

  printf("udatpg_getBestPattern returns %d -- %s \n", er, u_errorName(er));
  return 0;
}

Running the code:

LANG=C.UTF-8 LD_LIBRARY_PATH=~/icu/icu4c/source/lib ./test

Output:

udatpg_getBestPattern returns -127 -- U_USING_DEFAULT_WARNING

[srl295]

I can confirm that this fixed the crasher nodejs/node#27379 when floated as a patch against 64.2

[srl295]

@jefgen minor, but if you squash, can you mention the original commit and issue? something like fixes regression introduced in b12a927c9365bb38831afbf76fdd0999f8f33deb ICU-13778 (for history)

you could also make the first line of the commit a bit shorter. And i'll stop bikeshedding there.

[jefgen]

can you mention the original commit and issue?
you could also make the first line of the commit a bit shorter.

Thanks @srl295, those are good ideas. (I'll limit the first commit line to 70 chars max).

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