Allow registering internal hooks when variables are changed, e.g. LANG

[andychu]

• We need this for LANG=C support. See Test effect of locale environment variables on Oil #522
• Use case: validation
  • HOME=(an array) should be a runtime error?
• This would likely make some other things more efficient
• IFS? PS1? I think those have cached parsed representations so it's OK.

[andychu]

bash has a ton of these hooks in variables.c. We might only need a few

static struct name_and_function special_vars[] = {                                                                                                                                                                                            
  { "BASH_COMPAT", sv_shcompat },                                                                                                                                                                                                             
  { "BASH_XTRACEFD", sv_xtracefd },                                                                                                                                                                                                           
                                                                                                                                                                                                                                              
#if defined (JOB_CONTROL)                                                                                                                                                                                                                     
  { "CHILD_MAX", sv_childmax },                                                                                                                                                                                                               
#endif                                                                                                                                                                                                                                        
                                                                                                                                                                                                                                              
#if defined (READLINE)                                                                                                                                                                                                                        
#  if defined (STRICT_POSIX)                                                                                                                                                                                                                  
  { "COLUMNS", sv_winsize },                                                                                                                                                                                                                  
#  endif                                                                                                                                                                                                                                      
  { "COMP_WORDBREAKS", sv_comp_wordbreaks },                                                                                                                                                                                                  
#endif                                                                                                                                                                                                                                        
                                                                                                                                                                                                                                              
  { "EXECIGNORE", sv_execignore },                                                                                                                                                                                                            
                                                                                                                                                                                                                                              
  { "FUNCNEST", sv_funcnest },                                                                                                                                                                                                                
                                                                                                                                                                                                                                              
  { "GLOBIGNORE", sv_globignore },                                                                                                                                                                                                            
                                                                                                                                                                                                                                              
#if defined (HISTORY)                                                                                                                                                                                                                         
  { "HISTCONTROL", sv_history_control },                                                                                                                                                                                                      
  { "HISTFILESIZE", sv_histsize },                                                                                                                                                                                                            
  { "HISTIGNORE", sv_histignore },                                                                                                                                                                                                            
  { "HISTSIZE", sv_histsize },                                                                                                                                                                                                                
  { "HISTTIMEFORMAT", sv_histtimefmt },                                                                                                                                                                                                       
#endif                                                                                                                                                                                                                                        
                                                                                                                                                                                                                                              
#if defined (__CYGWIN__)                                                                                                                                                                                                                      
  { "HOME", sv_home },                                                                                                                                                                                                                        
#endif                                                                                                                                                                                                                                        
                                                                                                                                                                                                                                              
#if defined (READLINE)                                                                                                                                                                                                                        
  { "HOSTFILE", sv_hostfile },                                                                                                                                                                                                                
#endif                                                                                                                                                                                                                                        
                                                                                                                                                                                                                                              
  { "IFS", sv_ifs },                                                                                                                                                                                                                          
  { "IGNOREEOF", sv_ignoreeof },                                                                                                                                                                                                              
                                                                                                                                                                                                                                              
  { "LANG", sv_locale },                                                                                                                                                                                                                      
  { "LC_ALL", sv_locale },                                                                                                                                                                                                                    
  { "LC_COLLATE", sv_locale },                                                                                                                                                                                                                
  { "LC_CTYPE", sv_locale },                                                                                                                                                                                                                  
  { "LC_MESSAGES", sv_locale },                                                                                                                                                                                                               
  { "LC_NUMERIC", sv_locale },                                                                                                                                                                                                                
  { "LC_TIME", sv_locale },                                                                                                                                                                                                                   
                                                                                                                                                                                                                                              
#if defined (READLINE) && defined (STRICT_POSIX)                                                                                                                                                                                              
  { "LINES", sv_winsize },                                                                                                                                                                                                                    
#endif                                                                                                                                                                                                                                        
                                                                                                                                                                                                                                              
  { "MAIL", sv_mail },                                                                                                                                                                                                                        
  { "MAILCHECK", sv_mail },                                                                                                                                                                                                                   
  { "MAILPATH", sv_mail },                                                                                                                                                                                                                    
                                                                                                                                                                                                                                              
  { "OPTERR", sv_opterr },                                                                                                                                                                                                                    
  { "OPTIND", sv_optind },                                                                                                                                                                                                                    
                                                                                                                                                                                                                                              
  { "PATH", sv_path },                                                                                                                                                                                                                        
  { "POSIXLY_CORRECT", sv_strict_posix },                   

[andychu]

Note: dash has this. There's a function pointer in the variable binding structure that gets called upon mutation

[andychu]

• note TZ seems like it should behave like this, but it doesn't!
  • putenv() could be there? But you might need an unset hook too!
  • but tzset() can't be there? Because then it would affect it when NOT exported.

[andychu]

Somewhat related to #682 -- a variable change could be a hook?