Fix infinite loop from #21161 - repr of NumberFields (the parents) should indicate its embedding if there is one

[fchapoton]

Because this is causing some random infinite loops when running the testsuite. To reproduce, as reported in cschwan/sage-on-gentoo#541, use:

sage -t --long src/sage/structure/ src/sage/interfaces/

Another symptom:

The doctest

File "src/sage/structure/parent.pyx", line 1734, in sage.structure.parent.Parent.hom.register_embedding
Failed example:
    K.coerce_embedding()(a)

when it does not fail, and then one calls

K.coerce_embedding()

again, makes sage crash.

Removing the change of repr made in #21161 fixes that.

For the complete log when the doctest fails, see for example

https://patchbot.sagemath.org/log/27408/Ubuntu/18.04/x86_64/4.15.0-52-generic/petitbonum/2019-06-20%2014:50:38?short

CC: @embray @jdemeyer @videlec @jplab @JohnCremona @tscrim @mezzarobba @mkoeppe

Component: number fields

Author: Matthias Koeppe

Branch/Commit: c44fd16

Reviewer: Volker Braun

Issue created by migration from https://trac.sagemath.org/ticket/28036

[fchapoton]

comment:2

Adding the single line

gen = self.gen_embedding()

is enough to trigger the crash.

[fchapoton]

comment:3

This may be considered as a blocker, no ?

[timokau]

comment:4

Thank you for analyzing this issue! Previous discussion in cschwan/sage-on-gentoo#541, were we assumed it was distro related.

I agree that it should be included in 8.8.

[jplab]

comment:5

Replying to @fchapoton:

Adding the single line

gen = self.gen_embedding()

is enough to trigger the crash.

I do not get a crash with 8.8.rc2... :-S Hmm.

So there is no way to bypass the current issue and still be able to know via the repr that the NumberField is embedded? This is in itself problematic, right?

Reverting completely #21161 would be too bad... Isn't there any way to remove the crash caused by calling

gen = self.gen_embedding()

?

[fchapoton]

comment:6

Nota Bene: I got the crash/not crash when using py3-sage. Not tried with py2-sage.

[jplab]

comment:8

Replying to @fchapoton:

Nota Bene: I got the crash/not crash when using py3-sage. Not tried with py2-sage.

Hmm. I also tested with py3-sage. But I might have some obscure side effects...

[fchapoton]

comment:9

Here is a branch that fixes the crash.. Not sure if this is the right thing to do. And no idea if this prevents the random infinite loop to re-appear.

---

New commits:

c6c70e7

trac 28036 fix proposal

[fchapoton]

Branch: public/ticket/28036

[fchapoton]

Commit: c6c70e7

[fchapoton]

comment:11

the fix proposal provokes a few failing doctests, that may not be so simple..

[fchapoton]

Changed branch from public/ticket/28036 to public/ticket/28036_v2

[fchapoton]

Changed commit from c6c70e7 to fe00485

[fchapoton]

comment:12

Here is another proposal.. (not really convincing either)

---

New commits:

fe00485

another tentative fix

[mezzarobba]

comment:14

Replying to @fchapoton:

Here is another proposal.. (not really convincing either)

Did you test it? I don't understand how it can work, since _embedding is a cdef attribute. Apart from that, the idea looks sensible enough to me.

Another option may be to use a lazy string for the error message that causes the loop.

[mkoeppe]

comment:15

Replying to @mezzarobba:

Another option may be to use a lazy string for the error message that causes the loop.

+1

[sagetrac-git]

Changed commit from fe00485 to c44fd16

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

c44fd16

Use LazyFormat to fix infinite loop

[mkoeppe]

Author: Matthias Koeppe

[mkoeppe]

comment:18

I was able to reproduce this error. I agree it's a blocker. Using LazyFormat seems to fix it for me. Please test.

[mkoeppe]

comment:19

(Sorry for overwriting your branch.)

[fchapoton]

comment:20

This is a better solution indeed. Works fine for me (on py3-sage).

[dimpase]

comment:21

can that 1-line way to crash above be put into a doctest?

[mkoeppe]

comment:22

Replying to @dimpase:

can that 1-line way to crash above be put into a doctest?

I don't know how to reproduce the crash with a single doctest.

[fchapoton]

comment:23

add one line

K.coerce_embedding()

after the line

File "src/sage/structure/parent.pyx", line 1734, in sage.structure.parent.Parent.hom.register_embedding
Failed example:
    K.coerce_embedding()(a)

[mkoeppe]

comment:24

This triggers the error for me in an interactive session; but not if I add it to the doctest...

[vbraun]

Reviewer: Volker Braun

[vbraun]

Changed branch from public/ticket/28036_v2 to c44fd16