[binutils-gdb] libctf: adding CU mappings should be idempotent

[Nick Alcock]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=fdb4c2e02e6600b51f38a60cd70882887007cbdf

commit fdb4c2e02e6600b51f38a60cd70882887007cbdf
Author: Nick Alcock <nick.alcock@oracle.com>
Date:   Tue Nov 7 21:11:18 2023 +0000

    libctf: adding CU mappings should be idempotent
    
    When CTF finds conflicting types, it usually shoves each definition
    into a CTF dictionary named after the compilation unit.
    
    The intent of the obscure "cu-mapped link" feature is to allow you to
    implement custom linkers that shove the definitions into other, more
    coarse-grained units (say, one per kernel module, even if a module consists
    of more than one compilation unit): conflicting types within one of these
    larger components are hidden from name lookup so you can only look up (an
    arbitrary one of) them by name, but can still be found by chasing type graph
    links and are still fully deduplicated.
    
    You do this by calling
    ctf_link_add_cu_mapping (fp, "CU name", "bigger lump name"), repeatedly,
    with different "CU name"s: the ctf_link() following that will put all
    conflicting types found in "CU name"s sharing a "bigger lump name" into a
    child dict in an archive member named "bigger lump name".
    
    So it's clear enough what happens if you call it repeatedly with the same
    "bigger lump name" more than once, because that's the whole point of it: but
    what if you call it with the same "CU name" repeatedly?
    
    ctf_link_add_cu_mapping (fp, "CU name", "bigger lump name");
    ctf_link_add_cu_mapping (fp, "CU name", "other name");
    
    This is meant to be the same as just doing the second of these, as if the
    first was never called.  Alas, this isn't what happens, and what you get is
    instead a bit of an inconsistent mess: more or less, the first takes
    precedence, which is the exact opposite of what we wanted.
    
    Fix this to work the right way round.
    
    (I plan to add support for CU-mapped links to GNU ld, mainly so that we can
    properly *test* this machinery.)
    
    libctf/ChangeLog:
    
            * ctf-link.c (ctf_create_per_cu): Note the behaviour of
            repeatedly adding FROMs.
            (ctf_link_add_cu_mapping): Implement that behavour.

Diff:
---
 libctf/ctf-link.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/libctf/ctf-link.c b/libctf/ctf-link.c
index 27d11c97893..ac330629fc8 100644
--- a/libctf/ctf-link.c
+++ b/libctf/ctf-link.c
@@ -361,7 +361,8 @@ ctf_create_per_cu (ctf_dict_t *fp, ctf_dict_t *input, const char *cu_name)
 
 /* Add a mapping directing that the CU named FROM should have its
    conflicting/non-duplicate types (depending on link mode) go into a dict
-   named TO.  Many FROMs can share a TO.
+   named TO.  Many FROMs can share a TO, but adding the same FROM with
+   a different TO will replace the old mapping.
 
    We forcibly add a dict named TO in every case, even though it may well
    wind up empty, because clients that use this facility usually expect to find
@@ -371,7 +372,7 @@ int
 ctf_link_add_cu_mapping (ctf_dict_t *fp, const char *from, const char *to)
 {
   int err;
-  char *f = NULL, *t = NULL;
+  char *f = NULL, *t = NULL, *existing;
   ctf_dynhash_t *one_out;
 
   /* Mappings cannot be set up if per-CU output dicts already exist.  */
@@ -393,6 +394,19 @@ ctf_link_add_cu_mapping (ctf_dict_t *fp, const char *from, const char *to)
   if (fp->ctf_link_out_cu_mapping == NULL)
     goto oom;
 
+  /* If this FROM already exists, remove the mapping from both the FROM->TO
+     and the TO->FROM lists: the user wants to change it.  */
+
+  if ((existing = ctf_dynhash_lookup (fp->ctf_link_in_cu_mapping, from)) != NULL)
+    {
+      one_out = ctf_dynhash_lookup (fp->ctf_link_out_cu_mapping, existing);
+      if (!ctf_assert (fp, one_out))
+	return -1;				/* errno is set for us.  */
+
+      ctf_dynhash_remove (one_out, from);
+      ctf_dynhash_remove (fp->ctf_link_in_cu_mapping, from);
+    }
+
   f = strdup (from);
   t = strdup (to);
   if (!f || !t)