Change C APIs to a unified string implementation (#6299)

Currently in the entire C API of WPILib we have ~8 different ways of handling strings. The C API actually isn't built for pure C callers (We don't actually have any of those). Instead, they're built for interop between languages like LabVIEW and C# which can talk to C API's directly.

For output parameters, the choice was fairly obvious. An output struct containing a const string pointer and a length makes the most sense. Its easy to use these from most other languages, and doesn't require special null termination handling. Freeing these is also easy, as if you ever receive one of these string structures, theres just a single function call to free it.

Input parameters are a bit more complex. To be used from pure C, and from LabVIEW, a null terminated string is the best in most cases. However, null terminated strings in general have a lot of downsides. Additionally, from LabVIEW there are other considerations around encoding that having a wrapper struct helps make a bit easier. From a language like C#, a wrapper struct is by far the easiest, as custom marshalling can make it trivial to marshal both UTF8 and UTF16 strings down.

The final consideration is its nice to have an identical concept for both input and output. It makes the rules fairly easy to understand.

WPILib will not have any APIs that manipulate a string allocated externally. This means WPI_String can be const, as across the boundary it is always const.
If a WPILib API takes a const WPI_String*, WPILib will not manipulate or attempt to free that string, and that string is treated as an input. It is up to the caller to handle that memory, WPILib will never hold onto that memory longer than the call.
If a WPILib API takes a WPI_String*, that string is an output. WPILib will allocate that API with WPI_AllocateString(), fill in the string, and return to the caller. When the caller is done with the string, they must free it with WPI_FreeString().
If an output struct contains a WPI_String member, that member is considered read only, and should not be explicitly freed. The caller should call the free function for that struct.
If an array of WPI_Strings are returned, each individual string is considered read only, and should not be explicitly freed. The free function for that array should be called by the caller.
If an input struct containing a WPI_String, or an input array of WPI_Strings is passed to WPILib, the individual strings will not be manipulated or freed by WPILib, and the caller owns and should free that memory.
Callbacks also follow these rules. The most common is a callback either getting passed a const WPI_String* or a struct containing a WPI_String. In both of these cases, the callback target should consider these strings read only, and not attempt to free them or manipulate them.

wpiutil/src/main/native/include/wpi/DataLog_c.h

@@ -7,22 +7,12 @@
 #include <stddef.h>  // NOLINT
 
 #include <stdint.h>
+#include <wpi/string.h>
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
-/**
- * A datalog string (for use with string array).
- */
-struct WPI_DataLog_String {
-  /** Contents. */
-  const char* str;
-
-  /** Length. */
-  size_t len;
-};
-
 /** C-compatible data log (opaque struct). */
 struct WPI_DataLog;
 
@@ -33,9 +23,9 @@ struct WPI_DataLog;
  * @param errorCode error if file failed to open (output)
  * @param extraHeader extra header data
  */
-struct WPI_DataLog* WPI_DataLog_CreateWriter(const char* filename,
-                                             int* errorCode,
-                                             const char* extraHeader);
+struct WPI_DataLog* WPI_DataLog_CreateWriter(
+    const struct WPI_String* filename, int* errorCode,
+    const struct WPI_String* extraHeader);
 
 /**
  * Construct a new Data Log background writer.  The log will be initially
@@ -48,10 +38,9 @@ struct WPI_DataLog* WPI_DataLog_CreateWriter(const char* filename,
  *               this is a time/storage tradeoff
  * @param extraHeader extra header data
  */
-struct WPI_DataLog* WPI_DataLog_CreateBackgroundWriter(const char* dir,
-                                                       const char* filename,
-                                                       double period,
-                                                       const char* extraHeader);
+struct WPI_DataLog* WPI_DataLog_CreateBackgroundWriter(
+    const struct WPI_String* dir, const struct WPI_String* filename,
+    double period, const struct WPI_String* extraHeader);
 
 /**
  * Construct a new Data Log background writer that passes its output to the
@@ -67,7 +56,7 @@ struct WPI_DataLog* WPI_DataLog_CreateBackgroundWriter(const char* dir,
  */
 struct WPI_DataLog* WPI_DataLog_CreateBackgroundWriter_Func(
     void (*write)(void* ptr, const uint8_t* data, size_t len), void* ptr,
-    double period, const char* extraHeader);
+    double period, const struct WPI_String* extraHeader);
 
 /**
  * Change log filename.  Can only be used on background writer data logs.
@@ -76,7 +65,7 @@ struct WPI_DataLog* WPI_DataLog_CreateBackgroundWriter_Func(
  * @param filename filename
  */
 void WPI_DataLog_SetBackgroundWriterFilename(struct WPI_DataLog* datalog,
-                                             const char* filename);
+                                             const struct WPI_String* filename);
 
 /**
  * Releases a data log object. Closes the file and returns resources to the
@@ -134,9 +123,10 @@ void WPI_DataLog_Stop(struct WPI_DataLog* datalog);
  *
  * @return Entry index
  */
-int WPI_DataLog_Start(struct WPI_DataLog* datalog, const char* name,
-                      const char* type, const char* metadata,
-                      int64_t timestamp);
+int WPI_DataLog_Start(struct WPI_DataLog* datalog,
+                      const struct WPI_String* name,
+                      const struct WPI_String* type,
+                      const struct WPI_String* metadata, int64_t timestamp);
 
 /**
  * Finish an entry.
@@ -157,7 +147,8 @@ void WPI_DataLog_Finish(struct WPI_DataLog* datalog, int entry,
  * @param timestamp Time stamp (may be 0 to indicate now)
  */
 void WPI_DataLog_SetMetadata(struct WPI_DataLog* datalog, int entry,
-                             const char* metadata, int64_t timestamp);
+                             const struct WPI_String* metadata,
+                             int64_t timestamp);
 
 /**
  * Appends a raw record to the log.
@@ -221,11 +212,11 @@ void WPI_DataLog_AppendDouble(struct WPI_DataLog* datalog, int entry,
  * @param datalog data log
  * @param entry Entry index, as returned by WPI_DataLog_Start()
  * @param value String value to record
- * @param len Length of string
  * @param timestamp Time stamp (may be 0 to indicate now)
  */
 void WPI_DataLog_AppendString(struct WPI_DataLog* datalog, int entry,
-                              const char* value, size_t len, int64_t timestamp);
+                              const struct WPI_String* value,
+                              int64_t timestamp);
 
 /**
  * Appends a boolean array record to the log.
@@ -302,15 +293,18 @@ void WPI_DataLog_AppendDoubleArray(struct WPI_DataLog* datalog, int entry,
  * @param timestamp Time stamp (may be 0 to indicate now)
  */
 void WPI_DataLog_AppendStringArray(struct WPI_DataLog* datalog, int entry,
-                                   const WPI_DataLog_String* arr, size_t len,
+                                   const struct WPI_String* arr, size_t len,
                                    int64_t timestamp);
 
-void WPI_DataLog_AddSchemaString(struct WPI_DataLog* datalog, const char* name,
-                                 const char* type, const char* schema,
+void WPI_DataLog_AddSchemaString(struct WPI_DataLog* datalog,
+                                 const struct WPI_String* name,
+                                 const struct WPI_String* type,
+                                 const struct WPI_String* schema,
                                  int64_t timestamp);
 
-void WPI_DataLog_AddSchema(struct WPI_DataLog* datalog, const char* name,
-                           const char* type, const uint8_t* schema,
+void WPI_DataLog_AddSchema(struct WPI_DataLog* datalog,
+                           const struct WPI_String* name,
+                           const struct WPI_String* type, const uint8_t* schema,
                            size_t schema_len, int64_t timestamp);
 
 #ifdef __cplusplus