Add ISR print functions PrintFromISR() and VPrintFromISR()

The functions PrintFromISR() and VPrintFromISR() perform formatted
output from ISRs. Don't use them outside of an ISR. The functions do
not synchronize with the output of the regular print functions of any
task.

src/FormattedIO.c

@@ -3,6 +3,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "FormattedIO.h"
+#include <StrPrintf.h>
 #include <stdio.h>
 #include "Serial.h"
 
@@ -54,13 +55,93 @@ Print(const char* fmt, ...)
   return res;
 }
 
+typedef struct
+{
+  char* mBuf;
+  size_t mLen;
+} StrBuf;
+
+static int
+PutStrBuf(void* aParam, int aChar)
+{
+  StrBuf* buf = aParam;
+
+  if (!buf->mLen) {
+    return -1;
+  } else if (buf->mLen == 1) {
+    aChar = '\0'; /* always terminate string buffer */
+  }
+
+  *buf->mBuf = aChar;
+  ++buf->mBuf;
+  --buf->mLen;
+
+  return 1;
+}
+
 int
 VPrint(const char* fmt, va_list ap)
 {
   char buf[128];
-  int res = vsnprintf(buf, sizeof(buf), fmt, ap);
+  StrBuf strBuf = {
+    .mBuf = buf,
+    .mLen = sizeof(buf)
+  };
+  int res = vStrXPrintf(PutStrBuf, &strBuf, fmt, ap);
+  if (res < 0) {
+    return -1;
+  }
+  uint32_t len = sizeof(buf) - strBuf.mLen;
+  res = PrintIPC(len, buf);
+  if (res < 0) {
+    return -1;
+  }
+  return len;
+}
+
+static int
+PutSerial(void* aParam, int aChar)
+{
+  SerialPutChar(aChar);
+  return 0;
+}
+
+int
+_Print(const char* fmt, ...)
+{
+  va_list ap;
+
+  va_start(ap, fmt);
+  int res = _VPrint(fmt, ap);
+  va_end(ap);
+
+  return res;
+}
+
+int
+_VPrint(const char* fmt, va_list ap)
+{
+  int res = vStrXPrintf(PutSerial, NULL, fmt, ap);
   if (res < 0) {
     return -1;
   }
-  return PrintIPC(res, buf);
+  return res;
+}
+
+int
+PrintFromISR(const char* fmt, ...)
+{
+  va_list ap;
+
+  va_start(ap, fmt);
+  int res = VPrintFromISR(fmt, ap);
+  va_end(ap);
+
+  return res;
+}
+
+int
+VPrintFromISR(const char* fmt, va_list ap)
+{
+  return _VPrint(fmt, ap);
 }

src/FormattedIO.h

@@ -11,3 +11,17 @@ Print(const char* fmt, ...);
 
 int
 VPrint(const char* fmt, va_list ap);
+
+int
+PrintFromISR(const char* fmt, ...);
+
+int
+VPrintFromISR(const char* fmt, va_list ap);
+
+/* Internal functions for debugging; don't use in production code. */
+
+int
+_Print(const char* fmt, ...);
+
+int
+_VPrint(const char* fmt, va_list ap);