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Add recursion limit for string expansions
Unlike macros, REPTs and INCLUDEs, this recursion depth is independent.
This is intentional, because string expansions work very differently.

While it's easy to know when a string expansion begins, checking where it
ends is much more complicated, since the expansion's contents are simply
injected back into the lex buffer. Therefore, the depth has to be checked
after lexing took place.
Because of this, the placement of the expansion end check is somewhat
haphazard, but I think it's good. While I have no certainty, all tests
ended with all expansions properly ended, and I couldn't find any pitfalls.

Finally, `pCurrentStringExpansion` has been made global so error printing
can use it to tell the user if an error occurred inside of an expansion.

--- a/include/asm/fstack.h

+++ b/include/asm/fstack.h

@@ -35,7 +35,7 @@

 	uint32_t nREPTBlockSize;

 };

-extern unsigned int nMaxFileStackDepth;

+extern unsigned int nMaxRecursionDepth;

 void fstk_RunInclude(char *tzFileName);

 void fstk_RunMacroArg(int32_t s);

--- a/include/asm/lexer.h

+++ b/include/asm/lexer.h

@@ -40,6 +40,13 @@

 	LEX_STATE_MACROARGS

 };

+struct sStringExpansionPos {

+	char *tzName;

+	char *pBuffer;

+	char *pBufferPos;

+	struct sStringExpansionPos *pParent;

+};

+

 #define INITIAL		0

 #define macroarg	3

@@ -62,6 +69,7 @@

 void lex_Init(void);

 void lex_AddStrings(const struct sLexInitString *lex);

 void lex_SetBuffer(char *buffer, uint32_t len);

+void lex_BeginStringExpansion(const char *tzName);

 int yywrap(void);

 int yylex(void);

 void yyunput(char c);

@@ -70,6 +78,7 @@

 void yyunputbytes(uint32_t count);

 extern YY_BUFFER_STATE pCurrentBuffer;

+extern struct sStringExpansionPos *pCurrentStringExpansion;

 void upperstring(char *s);

 void lowerstring(char *s);

--- a/src/asm/fstack.c

+++ b/src/asm/fstack.c

@@ -29,7 +29,7 @@

 static struct sContext *pFileStack;

 static unsigned int nFileStackDepth;

-unsigned int nMaxFileStackDepth;

+unsigned int nMaxRecursionDepth;

 static struct sSymbol *pCurrentMacro;

 static YY_BUFFER_STATE CurrentFlexHandle;

 static FILE *pCurrentFile;

@@ -62,8 +62,8 @@

 {

 	struct sContext **ppFileStack;

-	if (++nFileStackDepth > nMaxFileStackDepth)

-		fatalerror("Recursion limit (%d) exceeded", nMaxFileStackDepth);

+	if (++nFileStackDepth > nMaxRecursionDepth)

+		fatalerror("Recursion limit (%d) exceeded", nMaxRecursionDepth);

 	ppFileStack = &pFileStack;

 	while (*ppFileStack)

--- a/src/asm/globlex.c

+++ b/src/asm/globlex.c

@@ -279,6 +279,8 @@

 	if (!oDontExpandStrings && sym_isString(dest)) {

 		char *s;

+		lex_BeginStringExpansion(dest);

+

 		/* Feed the symbol's contents into the buffer */

 		yyunputstr(s = sym_GetStringValue(dest));

--- a/src/asm/lexer.c

+++ b/src/asm/lexer.c

@@ -51,6 +51,9 @@

 uint32_t nFloating;

 enum eLexerState lexerstate = LEX_STATE_NORMAL;

+struct sStringExpansionPos *pCurrentStringExpansion;

+static unsigned int nNbStringExpansions;

+

 /* UTF-8 byte order mark */

 static const unsigned char bom[BOM_SIZE] = { 0xEF, 0xBB, 0xBF };

@@ -102,6 +105,31 @@

 	memcpy(pLexBuffer, s, len);

 }

+/*

+ * Marks that a new string expansion with name `tzName` ends here

+ * Enforces recursion depth

+ */

+void lex_BeginStringExpansion(const char *tzName)

+{

+	if (++nNbStringExpansions > nMaxRecursionDepth)

+		fatalerror("Recursion limit (%d) exceeded", nMaxRecursionDepth);

+

+	struct sStringExpansionPos *pNewStringExpansion =

+		malloc(sizeof(*pNewStringExpansion));

+	char *tzNewExpansionName = strdup(tzName);

+

+	if (!pNewStringExpansion || !tzNewExpansionName)

+		fatalerror("Could not allocate memory to expand '%s'",

+			   tzName);

+

+	pNewStringExpansion->tzName = tzNewExpansionName;

+	pNewStringExpansion->pBuffer = pLexBufferRealStart;

+	pNewStringExpansion->pBufferPos = pLexBuffer;

+	pNewStringExpansion->pParent = pCurrentStringExpansion;

+

+	pCurrentStringExpansion = pNewStringExpansion;

+}

+

 void yy_switch_to_buffer(YY_BUFFER_STATE buf)

 {

 	pCurrentBuffer = buf;

@@ -424,6 +452,9 @@

 	nLexMaxLength = 0;

 	nFloating = 0;

+

+	pCurrentStringExpansion = NULL;

+	nNbStringExpansions = 0;

 }

 void lex_AddStrings(const struct sLexInitString *lex)

@@ -968,12 +999,30 @@

 int yylex(void)

 {

+	int returnedChar;

 	switch (lexerstate) {

 	case LEX_STATE_NORMAL:

-		return yylex_NORMAL();

+		returnedChar = yylex_NORMAL();

+		break;

 	case LEX_STATE_MACROARGS:

-		return yylex_MACROARGS();

+		returnedChar = yylex_MACROARGS();

+		break;

 	default:

 		fatalerror("%s: Internal error.", __func__);

 	}

+

+	/* Check if string expansions were fully read */

+	while (pCurrentStringExpansion

+	    && pCurrentStringExpansion->pBuffer == pLexBufferRealStart

+	    && pCurrentStringExpansion->pBufferPos <= pLexBuffer) {

+		struct sStringExpansionPos *pParent =

+			pCurrentStringExpansion->pParent;

+		free(pCurrentStringExpansion->tzName);

+		free(pCurrentStringExpansion);

+

+		pCurrentStringExpansion = pParent;

+		nNbStringExpansions--;

+	}

+

+	return returnedChar;

 }

--- a/src/asm/main.c

+++ b/src/asm/main.c

@@ -315,7 +315,7 @@

 	/* yydebug=1; */

-	nMaxFileStackDepth = 64;

+	nMaxRecursionDepth = 64;

 	DefaultOptions.gbgfx[0] = '0';

 	DefaultOptions.gbgfx[1] = '1';

@@ -389,7 +389,7 @@

 			break;

 		case 'r':

-			nMaxFileStackDepth = strtoul(optarg, &ep, 0);

+			nMaxRecursionDepth = strtoul(optarg, &ep, 0);

 			if (optarg[0] == '\0' || *ep != '\0')

 				errx(1, "Invalid argument for option 'r'");