An overlong word (more than 50 characters) was encountered while indexing

Would this line still work correctly if we change the strlen() call to drupal_strlen()?

No, not at all. The problem is that our underlying database table uses a column of size 50, so there is really no way to correctly store such a long word there.
What we could do, though, I think, is make the maximum word length a setting (either hidden variable or in the server settings). I'd gladly accept such a patch (plus port to the D8 version, which has the same problem), but unfortunately I don't have the time to work on this myself.

You can however easily just "hack" the module and change "50" everywhere it occurs in the context of fulltext fields to a higher value.

Oh, seems I got that wrong. I'd always assumed the character limit set in the database would be a byte count, but you're right, it's actually a character count.
Under these conditions it's of course clear that you got the right solution here, we indeed should use the character count functions, not the byte count ones. However, it seems you missed another place in the same method. Please see and test my attached revised patch.

A test for this would also be great.
And we should apply the fix for string values, too.

I want to test the "tests_only" patch, too, please!

i have a same problem. pathc #7 didn't solved problem.

Apparently our default parsing in the Database backend split that "word" we were testing with into six separate words – and since I cannot read that script, I don't even know if that's correct or another bug. (Firefox and PhpStorm even fail me completely in rendering the string – only my terminal is able to.)

So, I think I'll use characters I can actually read (and have verified are not considered whitespace/punctuation by the backend) for testing.
Revised patch attached, please test/review!

@ arefen: Are you sure the word isn't actually longer than 50 (multi-byte) characters? If it is, my explanation in #2 applies.

You're right, forgot to update that comment.
And seems that while I explicitly checked that that string wouldn't be split further, it seems its lowercase variant still contained whitespace or punctuation. Changed the string to all-lowercase, now it should really work.

Since the test bot seems to be fine and no-one else complained, either: committed.
Thanks again for your help here!

Unless I'm mistaken, the D8 version has exactly the same problem.

Ported patch attached, including an additional test for string indexing.

Since the test bot agrees and no-one complained: committed.
Thanks again, everyone!

Just noticed that, while we did write a regression test method for this, it seems we somehow forgot to actually call it in our tests.
Even worse, for me locally, the tests failed when enabling it, so either the fix was wrong the first time, or we hit a regression – neither option a great prospect.
We have to enable this test and make it pass. (First step: check whether it would work with the module version from six months ago.)

Ah, thanks a lot for this fix!
However, seems the whole adding and removing of the body field is nonsense anyways, from some mistake several months back. Let's just get rid of that as well.

Looks great!

Good to hear, thanks for reviewing!
Committed.
Thanks again for your help, too, mfb!

Will there be any patch for Drupal 7?

This does not appear to be in the recent 7.x dev release. The bug does apply to D7, does it not?

Should the request for a drupal 7 patch be posted elsewhere? Is one in the works?
Thanks for your work on this.

This problem seems to still exist today.

An overlong word (more than 50 characters) was encountered while indexing: 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.
Since database search servers currently cannot index words of more than 50 characters, the word was truncated for indexing. If this should not be a single word, please make sure the "Tokenizer" processor is enabled and configured correctly for index Database Search Index.

I'm using Drupal 7(7.x-1.25) version. It seems like still, the issue exists.

This is the message in the watchdog log.

An overlong word (more than 50 characters) was encountered while indexing: ag9tcnz46rerustx2gbhtzyfaqh8yeqpjswaiqtaw3khruptwdxss.
Database search servers currently cannot index such words correctly – the word was therefore trimmed to the allowed length.

Any help, Thanks in advance.