cache_drop_all seg fault

Start with the reproducible IRB sequence in the issue, especially Vips.cache_drop_all followed by Vips::Image.new_from_file. Inspect lib/vips/operation.rb around line 225 and lib/vips/image.rb around line 288, then compare the GLib warnings with the crash stack. Done means the sequence no longer segfaults and the reported cache state is handled safely.

While trying to isolate my seg fault from https://github.com/libvips/ruby-vips/issues/429, I found another seg fault which is easy to reproduce:

require 'vips' 
path = "/Users/curi/w/test/test.png"
# vi1 = Vips::Image.new_from_file(path) # loads fine. irrelevant to crash
Vips.cache_drop_all # causes seg fault on next image load
vi2 = Vips::Image.new_from_file(path) # seg fault

macOS Tahoe 26.1, M1 chip, ruby 3.4.7, ruby-vips 2.3.0, vips 8.16.1 via homebrew.

irb(main):001> require 'vips' 
irb(main):002> path = "/Users/curi/w/test/test.png"
irb(main):003> vi1 = Vips::Image.new_from_file(path) # removing this doesn't change the result
irb(main):004> Vips.cache_drop_all # causes seg fault on next image load
irb(main):005> vi2 = Vips::Image.new_from_file(path) # seg fault

(process:41415): GLib-CRITICAL **: 13:19:13.336: g_hash_table_lookup: assertion 'hash_table != NULL' failed

(process:41415): GLib-CRITICAL **: 13:19:13.336: g_hash_table_lookup: assertion 'hash_table != NULL' failed

(process:41415): GLib-CRITICAL **: 13:19:13.336: g_hash_table_insert_internal: assertion 'hash_table != NULL' failed

(process:41415): GLib-CRITICAL **: 13:19:13.336: g_hash_table_lookup: assertion 'hash_table != NULL' failed
/Users/curi/.gem/ruby/3.4.7/gems/ruby-vips-2.3.0/lib/vips/operation.rb:225: [BUG] Segmentation fault at 0x0000000000000018
ruby 3.4.7 (2025-10-08 revision 7a5688e2a2) +PRISM [arm64-darwin25]

-- Crash Report log information --------------------------------------------
   See Crash Report log file in one of the following locations:             
     * ~/Library/Logs/DiagnosticReports                                     
     * /Library/Logs/DiagnosticReports                                      
   for more details.                                                        
Don't forget to include the above Crash Report log file in bug reports.     

-- Control frame information -----------------------------------------------
c:0025 p:---- s:0160 e:000159 CFUNC  :vips_cache_operation_build
c:0024 p:0005 s:0155 e:000154 METHOD /Users/curi/.gem/ruby/3.4.7/gems/ruby-vips-2.3.0/lib/vips/operation.rb:225
c:0023 p:0277 s:0150 E:000690 METHOD /Users/curi/.gem/ruby/3.4.7/gems/ruby-vips-2.3.0/lib/vips/operation.rb:481
c:0022 p:0072 s:0128 e:000127 METHOD /Users/curi/.gem/ruby/3.4.7/gems/ruby-vips-2.3.0/lib/vips/image.rb:288
c:0021 p:0029 s:0119 E:0018e8 EVAL   (irb):5 [FINISH]
c:0020 p:---- s:0113 e:000112 CFUNC  :eval
c:0019 p:0020 s:0105 e:000104 METHOD /Users/curi/.gem/ruby/3.4.7/gems/irb-1.15.3/lib/irb/workspace.rb:101
c:0018 p:0090 s:0098 e:000097 METHOD /Users/curi/.gem/ruby/3.4.7/gems/irb-1.15.3/lib/irb/context.rb:591
c:0017 p:0057 s:0090 e:000089 METHOD /Users/curi/.gem/ruby/3.4.7/gems/irb-1.15.3/lib/irb/context.rb:557