_dbm requires -lgdbm if gdbm is built as static libraries

[yan12125]

BPO	29440
Nosy	@ncoghlan, @yan12125
Files	gdbm.patch gdbm.patch: The correct patch, please ignore previous one

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-03-02.14:37:49.745>
created_at = <Date 2017-02-04.05:18:54.230>
labels = ['build', '3.7']
title = '_dbm requires -lgdbm if gdbm is built as static libraries'
updated_at = <Date 2019-03-02.14:37:49.741>
user = 'https://github.com/yan12125'

bugs.python.org fields:

activity = <Date 2019-03-02.14:37:49.741>
actor = 'yan12125'
assignee = 'none'
closed = True
closed_date = <Date 2019-03-02.14:37:49.745>
closer = 'yan12125'
components = ['Build']
creation = <Date 2017-02-04.05:18:54.230>
creator = 'yan12125'
dependencies = []
files = ['46507', '46517']
hgrepos = []
issue_num = 29440
keywords = ['patch']
message_count = 4.0
messages = ['286907', '286908', '286971', '337012']
nosy_count = 2.0
nosy_names = ['ncoghlan', 'yan12125']
pr_nums = []
priority = 'normal'
resolution = 'wont fix'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'compile error'
url = 'https://bugs.python.org/issue29440'
versions = ['Python 3.6', 'Python 3.7']

[yan12125]

In setup.py, _dbm links to gdbm_compat only. If gdbm is built as dynamic libraries, libgdbm_compat.so has a NEEDED flag for libgdbm.so, so both symbols in libgdbm and libgdbm_compat can be used. However, as static libraries does not provide such a flag, importing _dbm raises ImportError:

shell@ASUS_Z00E_2:/data/local/tmp $ python3.7m -c 'import _dbm'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ImportError: dlopen failed: cannot locate symbol "gdbm_errno" referenced by "/data/local/tmp/python3/usr/lib/python3.7/lib-dynload/_dbm.cpython-37m.so"...

gdbm_errno is a symbol in libgdbm.a.

gdbm manual [1] suggests linking to both libraries, too.

[1] http://www.gnu.org.ua/software/gdbm/manual/html_node/Compatibility.html

[yan12125]

Add the developer who wrote this line (bpo-15044)

[yan12125]

Sorry, -lgdbm should come after -lgdbm_compat, or symbols in libgdbm.a are still not resolved. I don't know why - linking is a magic :(

[yan12125]

I gave up building gdbm as a static library. It brings more headache than benefits.