Fix various compiler warnings and minor issues (#362)

List of changes:
- Fixed some -Wuninitialized warnings and removed some unused variables.

- Removed the unused extern for B_login (re: d8eba9d1).

- The libcmd builtins and the vmalloc memfatal function now handle
  memory errors with 'ERROR_SYSTEM|ERROR_PANIC' for consistency with how
  ksh itself handles out of memory errors.

- Added usage of UNREACHABLE() where it was missing from error handling.

- Extend many variables from short to int to prevent overflows (most
  variables involve file descriptors).

- Backported a ksh2020 patch to fix unused value Coverity issues
  (https://github.com/att/ast/pull/740).

- Note in src/cmd/ksh93/README that ksh compiles with Cygwin on
  Windows 10 and Windows 11, albeit with many test failures.

- Add comments to detail some sections of code. Extensive list of
  commits related to this change:
  ca2443b5, 7e7f1372, 2db9953a, 7003aba4, 6f50ff64, b1a41311,
  222515bf, a0dcdeea, 0aa9e03f, 61437b27, 352e68da, 88e8fa67,
  bc8b36fa, 6e515f1d, 017d088c, 035a4cb3, 588a1ff7, 6d63b57d,
  a2f13c19, 794d1c86, ab98ec65, 1026006d

- Removed a lot of dead ifdef code.

- edit/emacs.c: Hide an assignment to avoid a -Wunused warning. (See
  also https://github.com/att/ast/pull/753, which removed the assignment
  because ksh2020 removed the !SHOPT_MULTIBYTE code.)

- sh/nvdisc.c: The sh_newof macro cannot return a null pointer because
  it will instead cause the shell to exit if memory cannot be allocated.
  That makes the if statement here a no-op, so remove it.

- sh/xec.c: Fixed one unused variable warning in sh_funscope().

- sh/xec.c: Remove a fallthrough comment added in commit ed478ab7
  because the TFORK code doesn't fall through (GCC also produces no
  -Wimplicit-fallthrough warning here).

- data/builtins.c: The cd and pwd man pages state that these builtins
  default to -P if PATH_RESOLVE is 'physical', which isn't accurate:
     $ /opt/ast/bin/getconf PATH_RESOLVE
     physical
     $ mkdir /tmp/dir; ln -s /tmp/dir /tmp/sym
     $ cd /tmp/sym
     $ pwd
     /tmp/sym
     $ cd -P /tmp/sym
     $ pwd
     /tmp/dir
  The behavior described by these man pages isn't specified in the ksh
  man page or by POSIX, so to avoid changing these builtin's behavior
  the inaccurate PATH_RESOLVE information has been removed.

- Mamfiles: Preserve multi-line errors by quoting the $x variable.
  This fix was backported from 93v-.
  (See also <a7e9cc82>.)

- sh/subshell.c: Remove set but not used sp->errcontext variable.

src/cmd/ksh93/sh/args.c

@@ -642,7 +642,7 @@ void sh_printopts(Shopt_t oflags,register int mode, Shopt_t *mask)
  */
 char **sh_argbuild(Shell_t *shp,int *nargs, const struct comnod *comptr,int flag)
 {
-	register struct argnod	*argp;
+	register struct argnod	*argp=0;
 	struct argnod *arghead=0;
 	shp->xargmin = 0;
 	{

src/cmd/ksh93/sh/arith.c

@@ -272,7 +272,7 @@ static Sfdouble_t arith(const char **ptr, struct lval *lvalue, int type, Sfdoubl
 		c = mbchar(str);
 		if(isaletter(c))
 		{
-			register Namval_t *np;
+			register Namval_t *np=0;
 			int dot=0;
 			while(1)
 			{
@@ -297,7 +297,6 @@ static Sfdouble_t arith(const char **ptr, struct lval *lvalue, int type, Sfdoubl
 				int off=stktell(shp->stk);
 				int fsize = str- (char*)(*ptr);
 				const struct mathtab *tp;
-				Namval_t	*np;
 				c = **ptr;
 				lvalue->fun = 0;
 				sfprintf(shp->stk,".sh.math.%.*s%c",fsize,*ptr,0);

src/cmd/ksh93/sh/array.c

@@ -1775,7 +1775,7 @@ void *nv_associative(register Namval_t *np,const char *sp,int mode)
 				if((mode&NV_ADD) && nv_type(np)) 
 					nv_arraychild(np,mp,0);
 				if(sh.subshell)
-					np = sh_assignok(np,1);
+					sh_assignok(np,1);
 				/*
 				 * For enum types (NV_UINT16 with discipline ENUM_disc), nelem should not
 				 * not increased or 'unset' will fail to completely unset such an array.

src/cmd/ksh93/sh/deparse.c

@@ -74,7 +74,7 @@ void sh_deparse(Sfio_t *out, const Shnode_t *t,int tflags)
  */
 static void p_tree(register const Shnode_t *t,register int tflags)
 {
-	register char *cp;
+	register char *cp=0;
 	int save = end_line;
 	int needbrace = (tflags&NEED_BRACE);
 	int procsub = (tflags&PROCSUBST);
@@ -390,7 +390,7 @@ static void p_keyword(const char *word,int flag)
 static void p_arg(register const struct argnod *arg,register int endchar,int opts)
 {
 	register const char *cp;
-	register int flag;
+	register int flag=0;
 	do
 	{
 		if(!arg->argnxt.ap)

src/cmd/ksh93/sh/init.c

@@ -1063,16 +1063,6 @@ static char *setdisc_any(Namval_t *np, const char *event, Namval_t *action, Namf
 
 static const Namdisc_t SH_MATH_disc  = { 0, 0, get_math, 0, setdisc_any, create_math, };
 
-#if SHOPT_NAMESPACE
-    static char* get_nspace(Namval_t* np, Namfun_t *fp)
-    {
-	if(sh.namespace)
-		return(nv_name(sh.namespace));
-	return((char*)np->nvalue.cp);
-    }
-    static const Namdisc_t NSPACE_disc	= {  0, 0, get_nspace };
-#endif /* SHOPT_NAMESPACE */
-
 #ifdef _hdr_locale
     static const Namdisc_t LC_disc	= {  sizeof(Namfun_t), put_lang };
 #endif /* _hdr_locale */
@@ -1236,9 +1226,6 @@ Shell_t *sh_init(register int argc,register char *argv[], Shinit_f userinit)
 		shgd->lim.clk_tck = getconf("CLK_TCK");
 		shgd->lim.arg_max = getconf("ARG_MAX");
 		shgd->lim.child_max = getconf("CHILD_MAX");
-		shgd->lim.ngroups_max = getconf("NGROUPS_MAX");
-		shgd->lim.posix_version = getconf("VERSION");
-		shgd->lim.posix_jobcontrol = getconf("JOB_CONTROL");
 		if(shgd->lim.arg_max <=0)
 			shgd->lim.arg_max = ARG_MAX;
 		if(shgd->lim.child_max <=0)
@@ -1310,7 +1297,7 @@ Shell_t *sh_init(register int argc,register char *argv[], Shinit_f userinit)
 	sh_siginit(shp);
 	stakinstall(NIL(Stak_t*),nomemory);
 	/* set up memory for name-value pairs */
-	shp->init_context =  nv_init(shp);
+	shp->init_context = nv_init(shp);
 	/* initialize shell type */
 	if(argc>0)
 	{
@@ -1883,10 +1870,7 @@ Dt_t *sh_inittree(Shell_t *shp,const struct shtable2 *name_vals)
 		n++;
 	np = (Namval_t*)sh_calloc(n,sizeof(Namval_t));
 	if(!shgd->bltin_nodes)
-	{
 		shgd->bltin_nodes = np;
-		shgd->bltin_nnodes = n;
-	}
 	else if(name_vals==(const struct shtable2*)shtab_builtins)
 	{
 		shgd->bltin_cmds = np;

src/cmd/ksh93/sh/io.c

@@ -1135,7 +1135,6 @@ int	sh_redirect(Shell_t *shp,struct ionod *iop, int flag)
 	int r, indx = shp->topfd, perm= -1;
 	char *tname=0, *after="", *trace = shp->st.trap[SH_DEBUGTRAP];
 	Namval_t *np=0;
-	int isstring = shp->subshell?(sfset(sfstdout,0,0)&SF_STRING):0;
 
 	if(flag==2 && !sh_isoption(SH_POSIX))
 		clexec = 1;
@@ -1189,6 +1188,7 @@ int	sh_redirect(Shell_t *shp,struct ionod *iop, int flag)
 		}
 		if((iof&IOPROCSUB) && !(iof&IOLSEEK))
 		{
+			/* handle process substitution passed to redirection */
 			struct argnod *ap = (struct argnod*)stakalloc(ARGVAL+strlen(iop->ioname));
 			memset(ap, 0, ARGVAL);
 			if(iof&IOPUT)
@@ -1450,7 +1450,6 @@ int	sh_redirect(Shell_t *shp,struct ionod *iop, int flag)
 				else
 				{
 					regex_t *rp;
-					extern const char e_notimp[];
 					if(!(r&IOREAD))
 					{
 						message = e_noread;
@@ -2143,6 +2142,8 @@ static int	io_prompt(Shell_t *shp,Sfio_t *iop,register int flag)
 		flag = 0;
 	if(flag==0)
 		return(sfsync(sfstderr));
+	/* Temporarily disable 'set -o notify' while expanding the prompt to avoid
+	   possible crashes (https://github.com/ksh93/ksh/issues/103). */
 	was_ttywait_on = sh_isstate(SH_TTYWAIT);
 	sh_offstate(SH_TTYWAIT);
 	sfflags = sfset(sfstderr,SF_SHARE|SF_PUBLIC|SF_READ,0);
@@ -2165,7 +2166,7 @@ static int	io_prompt(Shell_t *shp,Sfio_t *iop,register int flag)
 			}
 #endif	/* TIOCLBIC */
 			cp = sh_mactry(shp,nv_getval(sh_scoped(shp,PS1NOD)));
-			shp->exitval = 0;
+			shp->exitval = 0;  /* avoid sending a signal on termination */
 			for(;c= *cp;cp++)
 			{
 				if(c==HIST_CHAR)
@@ -2199,7 +2200,7 @@ done:
 	if(*shp->prompt && (endprompt=(char*)sfreserve(sfstderr,0,0)))
 		*endprompt = 0;
 	if(was_ttywait_on)
-		sh_onstate(SH_TTYWAIT);
+		sh_onstate(SH_TTYWAIT);  /* re-enable 'set -o notify' */
 	sfset(sfstderr,sfflags&SF_READ|SF_SHARE|SF_PUBLIC,1);
 	return(sfsync(sfstderr));
 }

src/cmd/ksh93/sh/jobs.c

@@ -317,8 +317,7 @@ int job_reap(register int sig)
 	{
 		if(!(flags&WNOHANG) && !sh.intrap && job.pwlist)
 		{
-			if(!was_ttywait_on)
-				sh_onstate(SH_TTYWAIT);
+			sh_onstate(SH_TTYWAIT);
 			if(waitevent && (*waitevent)(-1,-1L,0))
 				flags |= WNOHANG;
 		}
@@ -1020,8 +1019,8 @@ int job_kill(register struct process *pw,register int sig)
 #endif	/* SIGTSTP */
 	job_lock();
 	errno = ECHILD;
-	if(pw==0)
-		goto error;
+	if(!pw)
+		goto error;  /* not an actual shell job */
 	shp = pw->p_shp;
 	pid = pw->p_pid;
 	if(by_number)

src/cmd/ksh93/sh/lex.c

@@ -1662,6 +1662,7 @@ done:
 	lp->assignok = (endchar(lp)==RBRACT?assignok:0);
 	if(lp->heredoc && !inheredoc)
 	{
+		/* here-document isn't fully contained in command substitution */
 		errormsg(SH_DICT,ERROR_exit(SYNBAD),e_lexsyntax5,lp->sh->inlineno,lp->heredoc->ioname);
 		UNREACHABLE();
 	}

src/cmd/ksh93/sh/macro.c

@@ -2161,7 +2161,7 @@ static void comsubst(Mac_t *mp,register Shnode_t* t, int type)
 		{
 			/* special case $(<file) and $(<#file) */
 			register int fd;
-			int r;
+			int r=0;
 			struct checkpt buff;
 			struct ionod *ip=0;
 			sh_pushcontext(mp->shp,&buff,SH_JMPIO);

src/cmd/ksh93/sh/nvdisc.c

@@ -842,11 +842,8 @@ static void *newnode(const char *name)
 {
 	register int s;
 	register Namval_t *np = sh_newof(0,Namval_t,1,s=strlen(name)+1);
-	if(np)
-	{
-		np->nvname = (char*)np+sizeof(Namval_t);
-		memcpy(np->nvname,name,s);
-	}
+	np->nvname = (char*)np+sizeof(Namval_t);
+	memcpy(np->nvname,name,s);
 	return((void*)np);
 }
 
@@ -1212,6 +1209,7 @@ Namval_t *sh_addbuiltin(const char *path, Shbltin_f bltin, void *extra)
 		{
 			if(nv_isattr(np,BLT_SPC))
 			{
+				/* builtin(1) cannot delete special builtins */
 				errormsg(SH_DICT,ERROR_exit(1),"cannot delete: %s%s",name,is_spcbuiltin);
 				UNREACHABLE();
 			}

src/cmd/ksh93/sh/parse.c

@@ -2046,7 +2046,7 @@ unsigned long kiaentity(Lex_t *lexp,const char *name,int len,int type,int first,
 		else
 			sfputr(stkp,name,0);
 	}
-	sfputc(stkp,'\0');
+	sfputc(stkp,'\0');  /* terminate name while writing database output */
 	np = nv_search(stakptr(offset),lexp->entity_tree,NV_ADD);
 	stkseek(stkp,offset);
 	np->nvalue.i = pkind;

src/cmd/ksh93/sh/path.c

@@ -1573,7 +1573,9 @@ static int path_chkpaths(Shell_t *shp,Pathcomp_t *first, Pathcomp_t* old,Pathcom
 	if((fd=open(stakptr(offset),O_RDONLY))>=0)
 	{
 		fstat(fd,&statb);
-		if (!S_ISREG(statb.st_mode)) {
+		if(!S_ISREG(statb.st_mode))
+		{
+			/* .paths cannot be a directory */
 			close(fd);
 			return 0;
 		}

src/cmd/ksh93/sh/shcomp.c

@@ -115,7 +115,7 @@ int main(int argc, char *argv[])
 	argc -= opt_info.index;
 	if(argc==0 && tty_check(0))
 	{
-		errormsg(SH_DICT,ERROR_exit(0),"refusing to read script from terminal",cp);
+		errormsg(SH_DICT,ERROR_exit(0),"refusing to read script from terminal");
 		error_info.errors++;
 	}
 	if(error_info.errors || argc>2)
@@ -161,7 +161,7 @@ int main(int argc, char *argv[])
 	if(vflag)
 		sh_onoption(SH_VERBOSE);
 	if(!dflag)
-		sfwrite(out,header,sizeof(header));
+		sfwrite(out,header,sizeof(header));  /* write binary shcomp header */
 	sh_offoption(SH_MULTILINE);
 	shp->inlineno = 1;
 #if SHOPT_BRACEPAT
@@ -174,6 +174,7 @@ int main(int argc, char *argv[])
 		if(t = (Shnode_t*)sh_parse(shp,in,0))
 		{
 			if((t->tre.tretyp&(COMMSK|COMSCAN))==0 && t->com.comnamp && strcmp(nv_name((Namval_t*)t->com.comnamp),"alias")==0)
+				/* Create aliases found in the script to prevent syntax errors */
 				sh_exec(t,0);
 			if(!dflag && sh_tdump(out,t) < 0)
 			{

src/cmd/ksh93/sh/subshell.c

@@ -73,20 +73,18 @@ static struct subshell
 	Shell_t		*shp;	/* shell interpreter */
 	struct subshell	*prev;	/* previous subshell data */
 	struct subshell	*pipe;	/* subshell where output goes to pipe on fork */
-	Dt_t		*var;	/* variable table at time of subshell */
 	struct Link	*svar;	/* save shell variable table */
 	Dt_t		*sfun;	/* function scope for subshell */
 	Dt_t		*strack;/* tracked alias scope for subshell */
 	Pathcomp_t	*pathlist; /* for PATH variable */
-	struct Error_context_s *errcontext;
 	Shopt_t		options;/* save shell options */
 	pid_t		subpid;	/* child process id */
 	Sfio_t*		saveout;/* saved standard output */
 	char		*pwd;	/* present working directory */
 	void		*jobs;	/* save job info */
 	mode_t		mask;	/* saved umask */
-	short		tmpfd;	/* saved tmp file descriptor */
-	short		pipefd;	/* read fd if pipe is created */
+	int		tmpfd;	/* saved tmp file descriptor */
+	int		pipefd;	/* read fd if pipe is created */
 	char		jobcontrol;
 	char		monitor;
 	unsigned char	fdstatus;
@@ -519,8 +517,6 @@ Sfio_t *sh_subshell(Shell_t *shp,Shnode_t *t, volatile int flags, int comsub)
 	sp->shp = shp;
 	sp->sig = 0;
 	subshell_data = sp;
-	sp->errcontext = &buff.err;
-	sp->var = shp->var_tree;
 	sp->options = shp->options;
 	sp->jobs = job_subsave();
 	sp->subdup = shp->subdup;

src/cmd/ksh93/sh/suid_exec.c

@@ -334,7 +334,10 @@ int eaccess(register const char *name, register int mode)
 			}
 			groups = (gid_t*)malloc((maxgroups+1)*sizeof(gid_t));
 			if(!groups)
-				error(ERROR_PANIC,"out of memory");
+			{
+				error(ERROR_SYSTEM|ERROR_PANIC,"out of memory");
+				UNREACHABLE();
+			}
 			n = getgroups(maxgroups,groups);
 			while(--n >= 0)
 			{

src/cmd/ksh93/sh/xec.c

@@ -175,7 +175,6 @@ static int      subpipe[3],subdup,tsetio,usepipe;
 
 static int iousepipe(Shell_t *shp)
 {
-	int fd=sffileno(sfstdout),i,err=errno;
 	if(usepipe)
 	{
 		usepipe++;
@@ -191,7 +190,6 @@ static int iousepipe(Shell_t *shp)
 
 void sh_iounpipe(Shell_t *shp)
 {
-	int fd=sffileno(sfstdout),n,err=errno;
 	char buff[SF_BUFSIZE];
 	if(!usepipe)
 		return;
@@ -1542,7 +1540,7 @@ int sh_exec(register const Shnode_t *t, int flags)
 				fifo_cleanup();
 #endif
 				if(shp->topfd > topfd && !(shp->subshell && (np==SYSEXEC || np==SYSREDIR)))
-					sh_iorestore(shp,topfd,jmpval);
+					sh_iorestore(shp,topfd,jmpval);  /* avoid leaking unused file descriptors */
 				exitset();
 				break;
 			}
@@ -1812,7 +1810,6 @@ int sh_exec(register const Shnode_t *t, int flags)
 				sh_done(shp,0);
 			}
 		    }
-		    /* FALLTHROUGH */
 
 		    case TSETIO:
 		    {
@@ -2899,7 +2896,6 @@ pid_t _sh_fork(Shell_t *shp,register pid_t parent,int flags,int *jobid)
 	if(parent)
 	{
 		int myjob,waitall=job.waitall;
-		shp->gd->nforks++;
 		if(job.toclear)
 			job_clear();
 		job.waitall = waitall;
@@ -2956,7 +2952,6 @@ pid_t _sh_fork(Shell_t *shp,register pid_t parent,int flags,int *jobid)
 	shp->outpipepid = ((flags&FPOU)?shgd->current_pid:0);
 	if(shp->trapnote&SH_SIGTERM)
 		sh_exit(SH_EXITSIG|SIGTERM);
-	shp->gd->nforks=0;
 	timerdel(NIL(void*));
 #ifdef JOBS
 	if(sh_isstate(SH_MONITOR))
@@ -3114,6 +3109,7 @@ int sh_funscope(int argn, char *argv[],int(*fun)(void*),void *arg,int execflg)
 	Dt_t			*last_root = shp->last_root;
 	Shopt_t			options;
 	options = shp->options;
+	NOT_USED(argn);
 	if(shp->fn_depth==0)
 		shp->glob_options =  shp->options;
 	else
@@ -3483,7 +3479,6 @@ static pid_t sh_ntfork(Shell_t *shp,const Shnode_t *t,char *argv[],int *jobid,in
 {
 	static pid_t	spawnpid;
 	static int	savetype;
-	static int	savejobid;
 	struct checkpt	*buffp = (struct checkpt*)stkalloc(shp->stk,sizeof(struct checkpt));
 	int		otype=0, jmpval,jobfork=0;
 	volatile int	scope=0, sigwasset=0;
@@ -3636,7 +3631,6 @@ static pid_t sh_ntfork(Shell_t *shp,const Shnode_t *t,char *argv[],int *jobid,in
 		if(grp==1)
 			job.curpgid = spawnpid;
 #endif /* JOBS */
-		savejobid = *jobid;
 		if(otype)
 			return(0);
 	}