PuTTY bug assert-line-not-null

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summary: Bombs out with line==NULL in terminal.c
class: bug: This is clearly an actual problem we want fixed.
priority: medium: This should be fixed one day.
present-in: 0.56 0.58 0.59

We receive occasional reports of PuTTY crashing out with line==NULL in terminal.c:lineptr(). However, we haven't been able to reproduce the problem ourselves.

In recent versions of PuTTY (0.58 and up), the error takes the form of one or more error boxes saying "line==NULL in terminal.c" and giving some diagnostic information, along with an exhortation to contact us.

In versions of PuTTY before snapshot 2005-03-07, the error is instead a failure of an assertion in terminal.c that line != NULL (the precise line number has changed a lot due to the extensive terminal re-engineering since 0.56).

We've had one report since we added diagnostics which indicates that screen doesn't contain enough entries. However, we haven't thought of a plausible mechanism for screen or alt_screen to end up undersized.

(Possibly related, although we've no hard evidence for this: assert-fail-newsavelines.)

There seem to be a lot of reports of this error from people using the serial backend in 0.59, which suggests that it can be a symptom of win-handle-double-free (fixed in 0.60).

In fact, in general, we suspect this is a symptom of some sort of memory corruption to which this assertion is particularly sensitive, rather than evidence of a direct logical error.

One user (1124367194.23192.240954800@webmail.messagingengine.com) reported a failure of the same assertion with PuTTY 0.58. However, as their screen is apparently zero rows high (presumably due to their window manager), this pathological case is a problem that's now protected against via zero-rows-cols-crash.

I am using bblean blackbox for windows as a shell on windows XP SP2, and whenever I rollup a maximized putty window the following fatal error results:

line==NULL in terminal.c
lineno=4621 y=0 w=93 h=0
count(alt=003B0F70)=0 alt_sblines=0
whichtree=00CFA2B8 treeindex=0

Perhaps an unrelated bug that's already been fixed:

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(last revision of this bug record was at 2017-04-28 16:52:45 +0100)